Another point is that, if you're un-armed and someone comes at you with a knife, there's a chance of defending yourself, even of disarming the assailant (and before the smart-ass small-dicks come in trying to say it can't be done, I've done it).

If you're un-armed and somebody's banging away at you from 20 yards with a gun, automatic or not, you're fucked and you're toast.