I haven't seen overhead stab even being used as much as underhand. Maybe this is different in duel matches, but as the idea of bayonet duels is rather ridiculous to me I wouldn't know. Group fighting is the major part of melee in this mod, and for that melee seems pretty good to me. I have played NW a lot since it was released, and have taken part in a few regiment training sessions and linebattles, and these claims of overhead stabs being dominant is at odds with what I've seen (just like the apparent surprise at the balance between swords and bayonets, as if it was better in M&M. If anything it was a bit worse for swordsmen in M&M. Where do these people play?). I also question whether it is the best choice for attacking cavalry; the overhead is more likely to hit the rider, but if he blocks or swerves you could do little or no damage to him or his horse. Lower stabs on the other hand get me most of my kills of riders by stabbing their legs, and you are bound to hit something, rider or horse. He can't block a low stab (if it is aimed beneath his lower block). I also find it less likely to whiff/bounce than the upper stab- that may be because I am not used to the range of the upper stab, or because it is more likely to hit the horse's head (which is closer than the body and so more likely to be too close to the stab).
Upper stabs are certainly different to lower stabs, but I haven't seen them being dominant, and so I disagree with the OP. That isn't to say that they shouldn't be refined, but I am worried by the essays that have been written by people insisting melee was perfect in M&M and taking on several people at once was a good thing that didn't ruin immersion etc. It seems some want the upper stab to be the same as the lower in every respect. It lacks the lower's ability to kill at point blank range- yes that can be overcome with turning the upper stab in a downwards trajectory, but that to me is an argument to make the lower stab as ineffective at point blank range, not the other way around. At least then some invention would be required rather than running through people and killing them with barely a millimetre of movement of the lower thrust.
@Vorlen: As it is, you can often avoid attacks by timing it right and moving to the side, if turn speed increases that will be less possible- the problem in M&M was, aiming badly didn't mean you would miss, you could thrust in one direction and turn almost 180 degrees and anyone caught in that entire arc would be likely to incur high damage or die.