Zooming in too much can negatively effect your gunnery.
It also greatly reduces your view of your surroundings. In effect you are detracting from any S/A you have if you stay in zoom view.
Many times when in a dog fight that hasn't really gotten underway yet, I'll zoom in to determine a planes direction, how he is banking ect, in order to determine my actions. But once the fight gets close, zooming in can really screw things up.