Well I kind of find the season finales or two-part-episodes as a refresher on the fact that she is still a pony who needs to have her work to do. While I agree that there is such thing is too less of appearances, and the same for too many. However, too many means that she would be the main character in EVERY episode. But when it comes to longer episodes, the point of view is mainly Twilight's since that's just her role as the main character and 2-part episodes take quite a long time thus Twilight takes a bigger part, more sentences, etc. As for the EG movies, we are actually seeing less and less of the pony Twilight and are just getting warmed up to Sci Twi