Ok let's do a clone debate, since, apparently, the majority of this community seems to have a problem with them. Let's see if Lucina is really a "step back" when it comes to good game design.
A clone widely viewed as a character, that is hardly even its own character (considering the effort it takes to make this character, excluding Toon Link) that promotes a different way to play the character it is cloning, is considered "boring" and shouldn't even be put in the game because of it.
I'm just going to remind you guys that what new players see on their character select screen is important. This is primarily in regards to Lucina's inclusion over her being an alternate costume:
New players don't try stuff out. They don't pay attention to characters like Zero Suit Samus in Brawl or Sheik in Melee if they are difficult to even load at the start of the match.
A character like Lucina or Toon Link being their own character would bring a LOT of appeal and would probably allow a game or series to get a lot stronger recognition than if they were being treated a mere costume.
Do you seriously think they would take any regard to Lucina if she was in the same slot as Marth? Wouldn't she just be, in their eyes, a female Marth just as Male Wii Fit Trainer is to the default female one?
Would anyone take Falco seriously in Melee if he didn't have an unique playstyle (and set of taunts) and was just placed under Fox?
Do you think anyone pays any respect to Daisy in her hinting with Peach's alternate palette? Do you think this would change if Daisy was an alternate costume?
I wouldn't think so.
Sakurai has created a really strong gimmick in these clones. His reasoning is more likely than not "different ways to play a character", which is ok in its own right in my opinion. But the clones that have stayed to this day have their own appeal outside of the gimmick that brought them on the table in the first place. They allowed the characters to be implemented as a, shall we say, "compromise" to appease the fan-base but not detract from the base of the game.
They are really popular. Popular enough to deserve a slot on the Smash Bros. Roster. Whatever what is own the character select screen means to is up to you.
There are certain characters that may need some working to differentiate themselves from their base (Toon Link), but this doesn't have to apply to every character.
Not every character needs an entirely different move-set. Having a different moveset does not make them "unique"; playing differently from the rest of the cast with a distinct style is what makes them unique.
Edit: I do not support Daisy being her own character, and her unpopularity stems from a lot of other issues. This was used primarily as an example, as her being a clone could have a possibility of changing some viewpoints against her and possibly bringing her more appeal from newer audiences, especially if she is designed correctly.
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