Dr. Mario has Mario's old moveset from 64 and Melee so it makes sense.
Lucina gives us a female option for Marth with different hitbox shit, so sure why not.
Dark Pit has stronger arrows and a better side-special. He's just better. Should have just given it to Pit instead and made Dark Pit an alt color. He was easy to make so that's why. (His theme song is awesome though, so awesome it's in the game twice).
If he was easy to implement then it's not a waste of a slot.