That's because AJ is best pony. <3
The notion that AJ could be "easily" omitted is ludicrous. The support she provides for the mane six cannot be taken over by any other character. Her team mom status brings a sense of equilibrium to the group, and she has historically been the one who has brought back the brink. Take The Ending of the End for instance, where all the mane six end up encouraging Twilight, but AJ is left as the one who really hammers the point home and snaps her out of her funk. Because that's simply her role.