I fail to see the practical difference between that and AC 1.
It has to do with the reckless charge, rage, and frenzy abilities. If you're allowed to go into the negatives, and it not be hindered, you could lower your AC as much as you wanted for as many bonuses as you wanted. If, however, you can't go into negatives, then you stop at 0, which means your bonuses are limited as well. Case in point, got a player with a character who has 7 AC, and would like to do all of the above. If the character can't go past 0, then the bonuses won't be there. Just looking for something that says yes or no.