Hussar
Legend
Sorry, I don't follow what you're saying at all. The actual rule-set is very important, since it is always a reflection of the game-world's physics. Granted, it's an imperfect reflection, so sometimes you need to fill in the gaps for niche situations that aren't explicitly covered, but it's consistent for everyone in for the things that it does cover.
If you have different rules for PCs and NPCs, then that status is necessarily observable in-game, which would be ridiculous. That's why it's important that, whatever the rules for disability, they apply equally to PCs and NPCs. I like the idea of covering broken limbs with just hit point damage, since that gives PCs an easy out with an in-game explanation: PCs have easy access to healing magic, which is why they never suffer from broken limbs or blindness for any significant period of time. Limbs that are missing entirely would be horrific enough that I'm happy to have them not show up within my fantasy game, even among commoners.
Even in 3e this wasn't actually true. Explain to me how a base Harpy, a medium creature that is pretty much physically the same as a human, starts off with 7 HD. Explain to me, referencing the 3.5 edition rules, how a harpy is 17 times tougher than an average human.