You really do take fictional characters way too seriously for your own good.
I almost feel sorry for you.
Anyway, Dark Pit has impact on the story:
- He opens the way to the Chaos Vortex. If it weren't for him, Palutena's soul would be lost forever.
- He helps Pit fight the Chaos Kin.
- He saves Pit's life twice.
About his victory theme, producing a short jingle isn't an hard thing to do. Especially when the composer that composed Dark Pit's theme (Motoi Sakuraba) also worked on this game.
If the anti-hero turning good is cliché'd, so is the hot-blooded hero (Pit), so is the tsundere (Viridi), so is the comical hammy villain (Hades). Character archetypes. Ever heard of them? Learn a thing or two about storytelling, then we'll talk.
As for the rest, it's all subjective. It's all YOUR opinion, so don't pass it as fact. It's really insulting.