Zelda is potentially already more unique with the Phantom attack than she ever was as Zelda / Sheik. Maybe not so in Melee, but definitely now! You said it removes something that makes her unique, I disagree fully. Zelda / Sheik was a reason to add the characters, but now Zelda has established herself as far more than her Ocarina of Time self. Her roles have became ever increasing, up to the point of getting playable herself, as Phantom Zelda.
Waluigi and Ridley are more hindered than Impa, cause unlike them, Impa could already use a base of an excisting character: Sheik. And replace her effectively. Waluigi also has almost nothing to base himself out of outside of a cluster**** of cameo apearance 'abilities' nobody really cares about, and Ridley is an extremely hard character to programm in the first place.
You may think it's stupid, but it happend before with Toon Link and Young Link. Even Mewtwo had a lot of his moves given to other characters. Take a good look at Charizard's Forward Tilt. Lucas's Down Smash. Lucario's Aura Sphere and Forward Smash. Wolf's Forward Aerial. Make your own conclusion out of that. Ike getting Eruption to despite never showing fire abilities...
Yet you insist it would be THE MOST RIDICULOUS THING EVER as so many around here do.
Most of you rely purely on some sort of psuedo logic saying : 'Expect what has been done' and sorry, I don't roll that way.