"Is it worth it?" is something that should also be considered about the creation of Echo Fighters.
Like, yes, you get Impa or Shadow or whoever, but once they're an Echo Fighter, like Ganondorf you've effectively doomed them to forever be a clone of the character they're copying, no matter how unique they could be in theory (and how many small alterations they get in future games).
People are still frustrated by the fact that Ganondorf and Dark Pit aren't (more) unique. Just because those characters have that "taint" doesn't mean it's justified to force that on other characters. That's getting into "an eye for an eye" territory.
Can Impa and Shadow be Echoes? Yes. Can they also be unique? Also yes. Should they be made Echoes just to make them Echoes to fill some arbitrary quota? Hell. No.
Lucina and Daisy have good reasons to be made (and kept as) Echoes, Lucina is defined by the fact she pretended to be Marth (and don't pretend that's not true), Daisy is (well, was) almost interchangeable with Peach (to the point that the Mario Bros. movie got them mixed up).
Dark Pit is more grey; he could, theoretically, use different weapons, but Dark Pit is also literally a clone of Pit, sharing the same memories, core personality, fighting style, and potential weapon arsenal.
The only remaining Echo Fighter I can fully get behind is Ken Masters, and that's only because he's a clone fighter of Ryu in Street Fighter.
Sorry for going on a rant there, but everyone going gaga for Echo Characters grinds my gears just a bit, and I needed to get this off my chest.