Same here, but again, Doc would be the one exception.
Doc compared to Mario:
Slower, more powerful, worse recovery, 2 different moves, pills.
Lucina compared to Marth:
Female FE rep, minor difference in sword balance (but a difference nonetheless), different attitude/personality.
Dark Pit compared to Pit:
Uh, not much actually. A different side special (yes, the properties are a bit different), can't get a very good angle on arrows as opposed to Pit, different attitude/personality.
Now, yes, there are more differences between Doc and Mario than the other two of the main three. However, Doc lacks one thing to actually be interesting. And that would be the one thing shared in common by the other two sets. Doc has ZERO personality. And because of that, I just shrug him off as a slightly modified version of Mario.
When I play as Marth, I feel like I can get up close to the opponent and rack up damage, and that I'm fighting for my people, representing something. However, Lucina is more vocal, so when I play as her, I feel like, it's either the opponent comes to me ("Come at me!") or I bring heck to them (oh ya, and I feel like I need to save something).
When I play as Pit, I feel like I'm fighting against evil, fighting with a good purpose. When I plays as Dark Pit, I feel I'm fighting to prove myself, and I feel more of a need to win.
When I play as Mario, I feel like I'm on a quest, and this is just an opponent that's in my way, that needs to be taken care of. When I play as Doc, I feel absolutely nothing. I don't feel like I'm fighting with a purpose, unlike the other clones.
It's boring.
In fact, I think I'd actually be alright with Lucina and Dark Pit being brought back for Smash 5. We need female FE representation, and Dark Pit may be the most similar of the clones, but his personality is the most different of the character he's cloned from. Though I dunno about either of their chances. I just hope Doc isn't in it.
Geez, that took a while for me to get out. But, I've dwelled on this subject for way too long. Let's not discuss it anymore and agree to disagree.