I honestly don't know how Riot decides when a theme should or shouldn't play at the start of the game. For me I think every time a champion gets their own theme ingame it adds a lot of flavor. Their themes represent their overall personality and stories very very well. If you were new to League and pick up Swain or Zoe for example and you would hear their themes at the start you would instantly know a lot more about these two without having ever heard quotes of them or read their stories. Swains sounds imposing and like an anthem, you instantly know this character is someone with great power and is a leader of sorts. Zoe on the other hand sounds calm, cosmical and playful, fitting for her childish nature and her reality warping powers. I really hope that, at some point, Riot decides to take all of the existing themes of champions or skins, cut them a bit short, maybe a little bit of editing like the ravens in Swains ingame theme and then add them to the start of the game. In the end it is just a little detail and sadly sometimes little details that are afterthoughts often aren't worth the time and effort they take that could be spent on things that have priority (New Champs, VGUs, Features and Story Art). Allthough something like this was already done when they gave old champions that had no recall animations just other animations they had as a recall, but I think that was more for gameplay clarity reasons than anything else.