Hey, I certainly don't mean it to sound like I'm giving you shit. (I agree - ALWAYS play through a song.) My right thumb bends back past a right angle because of whacking it on the pickup of my bass for a bunch of years. It hurts when it rains and everything.
It's just more like, take better care of yourself and learn to not hurt yourself on the instrument. Change string gauge, get the neck adjusted, hold the guitar differently, whatever. If you're really doing the Hetfield super-downpick thing, I'd say switch to bass picks and just plain old get your fingers further awau from the strings. It's not about holding back, maybe more about focusing.
I enjoy sloppy playing as much as anybody. I guess I just don't think extreme bodily harm is a part of it, is all. I reckon we're not playing the same type of music, though, so that's fine. I play light "easy" jazz.
If conviction/emotion/release=bleeding, then I think you're taking the guitar rock hero thing a little too seriously. I've bled a bit in my day on stage, but I don't see the point in pushing a performance to such an extreme that it renders you useless for the next gig. Unless your gigs are so few and far between that it doesn't matter (that's not a dig), where maybe balls-out is more justifiable. I'm just talking, you know... common sense, I guess.
I'm not saying you should cry, I'm saying a little electrical tape can do wonders. The only people that should bleed that much in front of an audience are performance artists, because they crave the attention that much.