It just become a useless spell, at low level +2 won't do much cause only 10% chance per attack it can turn a hit to miss; at high level +6 also won't do much due to 5e character's AC don't increase much, but monster's attack bonus increase A LOT cause monster ability score can be 20+ and their CR often higher than party's level so their PB is also higher.
EDIT: So it will only be useful when used by optimizars who make AC already very high to impossible to hit without crit. I know its current form already is, but change it to +PB hurt normal player more than optimizars.