The head-on shot is an important part of real ACM. If the threat of it does not exist, a large part of ACM is changed from what it was in reality, especially merges.
Also, in the extreme case of head-on's not working at all, you get totally unrealistic fights, like Tumor says. You can flounder around at stall, a sitting duck, but as long as you get your nose to within 45 degrees of an enemy, he can't shoot you.
A Storch becomes unkillable.
-- It takes as much skill to land head-on shots as it does to land any other shot.
-- If you have maneuvering speed, you can avoid head-on hits from the large majority of AH pilots.
-- If you don't have maneuvering speed, you don't deserve to be saved just because you got your nose pointed vaguely in the direction of the attacker.