If you use fumbles to roll a natural 1 is to fumble yet what does this mean for the system as presented? Anything thing with multiple attacks a round is more likely to fumble! That fighter you finally leveled to the point that he can attack twice a round? Compared to a peasant he is almost twice as likely to fumble an attack! It just gets worse from there on. A character that can make multiple attacks in a round should be skilled enough to fumble LESS then before not more. That is the whole idea behind levels, the higher you are the better. Of course it is the same for a critical hit but that at the least follows the level scheme.
Every roll of the d20 has a 5% chance to land on a specific number. While this is a stable chance with each roll being just as likely the fact that every roll you make has that chance of coming up as a 1 it adds up and with two attacks a round your chance of a fumble for that round is 9.75%. My personal feelings are just to get rid of crits and fumbles but they can be fun for both flavor and mechanic standpoints. I have put some thought into how I could have them and have "Fun" outcomes for fumbles yet not penalize a fighter for being good. What I came up with was copying the critical confirmation from 3rd edition but in reverse. For a fumble you need to get a natural 1 and roll a second check and fail to hit the target again just like to confirm a critical you need to roll another attack and hit. This would make it so that a fighter who can handle his sword will be less likely to fumble against a weaker opponent. A good way to spin it is as a fumble recovery check representing a experienced combatants ability to recover from a mistake, after all even if a 10 level fighter does go off balance he should be able to recover from it when fighting against a less skilled combatant.
With the fumble recovery check it also allows you to implement "Better" fumble results and not feel to guilty about it. You would feel bad if the high-level fighter takes a swing at some kobold and manages to disembowel himself with just a single roll of the dice. With the check you can include such things though still in moderation because the fighter gets a chance to save himself and if it takes a 1 to miss a creature and they roll it twice in a row, well the Dice have spoken.