Y'know, I've been curious about Gems missing body parts for a bit now too.

Sure, their bodies are illusions made of light, but they still need them in a physical sense. Especially since, as far as we know, it can take a while for a gem to reform. (I think Amethyst's speed is an exception, but Ruby and Sapphy did too in Jailbreak.) I guess it also depends on how much damage a gem can take before they poof. They can handle blunt trauma to an extent, but all Pearl got was a simple stab, with no vital organs to speak of, and she instantly went out.

I just think about maybe during the war, you've got Gems maybe missing a limb or a chunk of their body but keep going at it. Since it might take a long amount of time to reform if they poof themselves, and they don't want to leave their comrades hanging. Or maybe a Gem gets cracked. They don't fall apart like Amythest did, but they can't reform either, so they just keep fighting on and getting more and more injured until they're finally taken out.

A hero denied her Pearl.