To expand on an earlier point, I remember an anime example I read on TV Tropes (but can't find it there at the moment).
There was an anime TV series where the mother of one of the main characters was never present -- except for one episode. She, being competent and capable, completely blocked and resolved all the crazy antics that usually happened in the series well before they became plot points. At the end of the episode, she left again and things returned to normal.
In other words, having Princess Celestia actually doing her job or Applejack and her common sense being more available (or for that matter having a Royal Guard, Wonderbolts, Mayor, or other authority that can actually handle the typical episode goings-on) undermines the writers' ability to present a simple story or show off other characters.