I don't like that spell, either. Some things even magic can't justify. I'm not even calling it broken, it's just bad. A full-attack in melee represents an exchange of blows, of which 1-4 actually have a chance of hitting. An AoO is an attack that only has a chance of hitting because the opponent got distracted. A full-attack with a bow represents firing 1-4 arrows. An AoO with a bow represents firing an extra arrow because...the opponent...isn't paying attention to you? Not broken, just thematically it doesn't work.