A glancing thrust will be much more damaging with a hewing spear.

Also some speculation from my personal practice with cutting polearms, the ability to perform a draw cut has a lot of value, as it comes into play when the spearman is vulnerable, meaning while pulling back the weapon after a missed thrust.

I always thought it was mostly to make it harder to grab. Despite being extremely effective when matched up against other hand weapons, spears are also ironically the easiest threat to completely defuse if the opponent can manage to grasp the shaft, and thus the spear probably less dominant against unarmed opponents than axes, swords, and maybe even daggers are. Giving it a long sharp tip you can't safely grab onto might simply be an attempt to mitigate this weakness in cases where dominating unarmed or lightly armed opponents is more important. Seems like a good idea in guard duties, which might explain why the concept appears again in later partisans. If the heavier tip has other advantages like making it harder to parry and bind against, all the better.

The importance being overlooked related to the difficulty of parrying it with the added weight is simply that, it aids in the energy of the blows themselves and potential damage, especially with cast thrusts, vs lighter weapons. The overarm technique with spears is suited mostly for striking at faces, necks, and the upper torso. If armor is present, the upper torso is entirely a no-go with coverage of haurberks. This leaves the nape of the neck, the neck, and the face with typical helms of the time. A strike from a hewing spear will both be higher energy, thus assisting in damage to the skull, and ensure even glancing thrusts will still deal damage. A gliding thrust which misses the full impalement of the neck will also quite likely, still cut deep and maybe even sever an artery.

Another thing to consider is status, especially in the Romanizing fetishes of the Carolingians. Menaulion were great spears used by the Romans at this point of time, long two handed ones made of a small tree tipped with a spearhead as long and broad as a small sword, meant for felling horses. Mere imitation of this might have been done simply for prestige.

Speaking to the last point: In the Icelandic sagas duels with the atgeirr (which is sometimes translated as "spear-axe," but is probably just a poetic term for a warrior's heavy spear - whether hewing or winged or not - as opposed to a freeman's more flimsy spear) is often depicted just like that. The combatants start with spear and shield, but then the shield is damaged and discarded, and the dramatic finish is delivered with the spear in both hands.

5:15 Correct, an improper thrust becomes a cut. A circular or ovular cut action would be effective even as enemies parry the thrust. If a Swiss box or something similar was being deployed, this would be a great weapon for first line. Slice wrists, armpits, faces, necks 'in the bind' as the long pikes thrust forward.
Likewise this slicethrust would assist in hamstringing cavalry.

I think just any amount of cutting edge on a spear gives much of an advantage.  It doesn't need to be a saber and have the sheering power, it just gives the weapon more surface area to threaten or harry with.  You don't need to draw back for a full swing to effectively do harm, a quick slap to the side of the head or the face of the opponent without a helmet or with an open face helm is still going to be debilitating even at a few inches of weapon travel.  It's just better lateral threat.

Probably not intentional, but if the shaft gets broken at any length, you still have a short-spear or short sword to work with

The worst thing that can happen to a spearman is the enemy grabbing his weapon and pulling it out of his hand. If the blade is small, grabbing the spear is easy, but if the blade is long and wide, the enemy has nothing but blade to grab. The next worst thing is the blade getting stuck in somebody's ribs, which is much more likely to happen if the blade is small enough to completely sink into the ribcage. By having a long, wide blade, you reduce the probability of this happening. The next worst thing is somebody lopping off your spearhead, which is more likely to happen, when your blade is small and your haft thin and light. When your blade is large, the enemy is more likely to hit the blade than the haft. If your haft is thick, as it probably is when the spearhead is large, cutting through it is that much harder. Of course, it's also good to be known as the man with the biggest spear in town, especially among the ladies.

The first strike is usually the most important, and if you are guarding a princes court you might want a weapon that works from an upright ready position that can be held comfortably for hours.

Something worth mentioning is how hard it would be to disarm somebody wielding a hewing spear as opposed to a conventional one.
The length, width, and taper profile of the hewing spearhead makes it nearly impossible to grab in the heat of battle without seriously injuring your hand or getting stabbed in the process. The long cutting edge forces you to step toward the enemy or fully extend your arms to reach the wooden shaft behind the spearhead, whereas a quick thrust or pull of the spear would viciously slice up the hands of anyone gripping the blade itself, thanks to the palm-width taper halfway down its length.

For crowd control, the unruly mob of peasants trying to overthrow your lord might be less likely to grab your spear head and take it away from you and shove it up your arse.

I have been training with and using a 9 ft hewing spear for many years now and I have tested it against a wild boar (Russian wild boar / American domestic pig halfbreed) that I took with a boar spear (which also has a wide but shorter blade). The answer is leverage. A sword is powerful but relies on draw cuts to transfer power. A hewing spear is a very wide sword on the end of a long lever. That leverage produces insane power on contact and with a very sharp blade I was able to hew deep into the hog and then draw cut even deeper by retracting the spear in my grip (chambering for a thrust with great violence of force while the spear was deep in the body of the animal). Hewing spears are insanely good against unarmoured or lightly armoured opponents. Yet they maintain their thrusting power surprisingly well despite being somewhat more difficult to fence with. Despite it's shortcomings for certain applications it remains my favorite spear and really inspires fear when I demonstrate it's deadly potential to people interested in getting into spear as a weapon for martial arts purposes. After I gut a pig with a single swing they have respect for the weapon and handle it with care, though most of the time the demonstration is on large mellons. Even just a simple low speed short length of travel swing will cleanly cut the mellon in half.

My first thought is that that shape isn't necessarily bad for thrusting either. Could it just be that they wanted broader heavier blades while keeping a very stabby tapered blade profile? If you just take a small spearhead and stretch it to make it wider you eventually end op with a kind of blunt shape, making it longer would be a natural solution.

Pair a unit of these with a couple of Dane-axes in the mix and this could definitely wreck a charge.

Because the melee' is not single combat: you take what you can get.

"cutting power minimal" wth are you talking about?? The speed the end of that spear will get to will be insane. They cut VERY well. As good or better than an arming sword. If your opponent is unarmored or you can attack an unarmored location this will do alot of damage with high stopping power, which, in wars, matters alot. Thrusts are lethal, but can cause slower deaths, are less useful for maiming arms or legs, and generally cannot break open someone's skull. A larger bladed spear is much more likely to do so. And finally, thrusts with a larger bladed spear will also have more stopping power. You are trading speed and maneuverability for lethality. Much worse at stabbing through armor, but still able.

I always thought of it as a half way glaive spear

I don't buy the binding argument for two reasons. For two-handed use, your hands are so far apart you can overcome all but the most severe disadvantages (e.g. a pollaxe) by leverage alone. For one-handed use, there are so, so many more important factors to inertia than the head used - how thick is your shaft and what wood is it made of being the most important. And even then, if you have a spear and a shield, you are better off parrying with a shield a lot of the time anyway, especially with most early medieval shields (e.g. center grip or small round strapped).

(Do note that pollaxe is not early medieval)

The one reason you didn't mention is durability - I used to have spear shafts made of beech or ash wood, and they got chewed up like hell, lasting maybe a year of monthly-ish use, and that's going against blunt swords. I switched to acacia wood, aka the arboreal adamantium, but the shaft near the tip is still getting damaged as time goes on.

(Do note that acacia, while early medieval, is only growing in America at this point)

You could fix that durability issue with langets, pollaxe-style, but that is just as much if not more metal than just making that bladed bit longer and giving you the ability to do some slices when you miss once in a while.

(Slicing a neck artery of someone you don't like is timeless, and therefore early medieval)

I've always thought of spatulated spear heads as being designed to make a huge wound channel on lightly armored and unarmored targets in the thrust as in boar spears. The more spatulated spear heads show up in the age of pyke and shot once armor was becoming obsolete due to gunpowder. Those spears heads are also great at cutting with draw or "pull" cuts.

A concrete effect of the very long blade is that it makes it harder for the opponent to grab safely. It increses the distance where he can grab.
To achieve this you only need to make the blade longer (as in the hewing spear), you do not need to make it wider. So it maps quite well to the design.