From what I've read though, the most weighty argument I've seen against critical fumbles is this:
- most NPCs won't be around to experience even one of them.
- the player characters will experience tons of them.
This means critical fumbles penalise the players more than help them. It also means that critical fumbles with any long-term consequence (like a character losing a limb or a weapon) exclusively impacts the players -- the NPCs won't be around long enough to care about the long-term consequences, so exclusively only the players actually suffer from them.