The better the gunery is the less important ACM becomes. Thus, theoretically, gunnery > ACM in real life, as well as the game.
Just anticipate the flightpath and shoot where the guy will arrive, and then you don't even have to 'pursuit' the enemy at all. Everybody has their moments when they pull off a really terrific, blind-lead deflection shot, where you don't maneuver at all, just see the enemy plane coming, aim at a certain void in the air, and then boom! you got him.
The problem is, some people can pull that off relatively easily, while others cannot. Then you have to start the whole 'chasing' sequence of cutting the edges, driving the target into a corner, outmaneuvering him, and etc etc..
Since one cannot just flat-out, deflection shoot the target outta the sky, anytime, he has to chase him around and constantly maintain a certain amount of visual info on the target.
Thus, when gunnery is lacking, or simple gunnery passes fail, then comes the ACM part to make up for the screw-up in initial gunnery attempt.