I'm all for crippling AI Controlled. As long as it doesn't happen to the player.
It would get really annoying to constantly have to heal crippled limbs limiting your movement when you get hit. Really. I don't want to keep a healing spell at all times either...
I can foresee serious rage playing and being stuck/slow as hell due to a crippled limb, being unable to move as you please... Worst thing ever in a game.
If my limbs were crippled and I couldn't make it to a healer, I would enjoy playing as a cripple
Frequently in New Vegas hardcoe mode all of my limbs were crippled for long periods of time and I had to hobble around to find a doctor.
Regarding the suggestion for a spell to heal crippled limbs, I suppose it makes sense if there are spells to cure diseases, etc., but IMO it would be best if the spell was only available at, say 100 restoration, and was quite expensive. If it were available cheaply at level 1 there would be no point to including crippled limbs in the game.