What is the drawback of turning it off?
The biggest drawback is that to point your nose at an enemy aircraft and maneuver into him such that you fly right through him while holding down your trigger requires no skill at all- it completely eliminates the second hardest part of getting a gunnery solution, which is to not kill yourself while you are at it.
Right now, the guys who TRY to hold down the trigger and fly right through you usually die in the process - they might kill you some of the time but they usually die in the immanent collision almost all of the time. Like lab rats, they then start to learn that they probably need to do something different in order to survive the engagement.
But what you are proposing is to take away the only downside in order to "resolve" a relatively minor issue where we currently accept a less than perfect game experience in exchange for the ability to play with other people at disparate worldly locations - an issue that only mostly the uninformed argue is a problem to begin with.
It would destroy any semblance of realism - real life defensive maneuvers would work poorly at best, and realistic attack methods would be far less effective than the totally artificial "collision gunnery pass".
In short, it totally suck...
That's why!
EDIT -> Lusche keeps beatin' me to it!