The long spears ... well, M&B wasn't really designed with long spears in mind. That we can't fix.
Having guys with sticks beat on your armor for a while ... well, that happens. Normally there's somebody in the group with an axe or something, but if not - well, the same thing happened to some cops in riots over a WTO meeting somewhere a few years ago. A bunch of angry protesters beat on a couple of them for what looked like forever, unable to really hurt them through all the body armor, but not letting them get up. That's part of armored combat. Unfortunately, M&B does not include a surrender button. (Those cops at the WTO riot tried to surrender too, but nobody was listening.)
Tragically, the AI cannot really be changed within the scope of the mod script. The closest we came was giving a lot of low-end units some rocks to throw, so they wouldn't always follow your horse around like a bunch of baby ducks.
Don't let them corner you. If you ride into a whole herd of guys and let them knock you down, then you deserve to get used as a punching bag.
Seriously, these are things that can't be fixed. We already discussed tougher bandits, i.e. less fools with sticks and more real weapons.