Been playing some Fallout 3? :) I just finished it last night, but it was sort of bittersweet as I sort of rushed through the end to ensure I could focus on Wrath. Yeah, I know, I play WoW. I'm a traitor or whatever. IT'S A FUN GAME!
:)
Anyway... It's a cool idea, but yeah, they probably won't be around long enough to actually get any use out of it. We're obviously putting a greater emphasis on varying mechanics on enemies and trying to make them interesting to fight and 'game', but the game is still about pulling your sword through a massive huddle of demons. Something as complex as a limb-specific crippling ability probably just wouldn't get enough screen time to make it worth it.
I could see it more as a potential mechanic for progress in a boss fight maybe? Something that's a bit more substantial in health/fight length and also size and visibility of something like a crippled limb. I don't know, it's cool, but whether it would actually work out or not is debatable.