We use the same system in my group (a "1" is a fumble threat), but I'm wondering if it really does work in favor of high-BAB classes like Fighters. Considering that extra attacks from high BAB have successively lower to-hit bonuses, having more than one attack is still a bit of a penalty. Say an 11th level fighter rolls a 1 on his last attack - his BAB for that attack is only +1, the same as a 1st level fighter. Now the 11th level fighter probably has all kinds of bonuses to hit that a 1st level one would not, but his foes also have much higher ACs than what a 1st level fighter would see.
I think I'm starting to see why the designers left fumbles out of the game.
I think I'm starting to see why the designers left fumbles out of the game.