Gimps fighters by making them unable to fight.
The other side of the argument is that it makes fighters fight smart. No longer can they wade into untold hordes without really worrying about being hurt, spreading death and mayhem in their path. I'm thinking of the scene in LotR: TTT where Aragorn and Gimly jump off the battlements at Helm's Deep onto the bridge into a sea of orcs and goblins.
A rule like this wouldn't be one for the super heroic games who want to emulate that scene. But, for the GMs who see that scene and say, "Riiighht. Not in my game," this rule might be useful.
Instead of just wading in, fighting one-on-one (or one-on-nine), and doing the deed, this rule would make a fighter's player think more strategically and tactically.
Instead of, "Hey! There's 12 Orcs! Let's go fight 'em!", the rule would encourage a player to think more along the lines of, "Hey! There's 12 Orcs! I'll get their attention and draw them back here. You two spring an amush from the sides, and we'll catch 'em by surprise."
You know what would really make the game more gritty? Making death meaningful (remove Raise Dead etc.)!
I play the Conan RPG. No Raise Dead spell in that game. When your character dies, he's toast.
Do you actually know anybody who really LIKES 'gritty'?
I do. My group does. I see another poster popping up that says he does. And, there are several supplements for sale, like the one I got this idea out of, that are designed to make the game more gritty and realistic. Somebody is buying these supplements.
All 'gritty' rules ever seem to do is make the game harder, more deadly, and consequently less enjoyably for somebody who likes his character to survive and accomplish something.
Harder? That's a point of view. Definitely makes you think, more.
More Deadly? Certainly.
Less Enjoyable? I don't think so. I think a certain amount of grittiness enhances the game. I do understand, though, that some players are interested in playing BIG DAMN HEROES, and I know that can be a hoot, too. It's just different play styles. Star Wars (Big Damn Heroes) is cool, but so is Aliens (Gritty).