What good does increasing the roster size do if they mostly play the same? It's just redundant to have them if they offer nothing new. I could understand Roy because his attacks are stronger at the hilt but Lucina and Chrom offer nothing unique. Lucina is just Marth without a tipper and Chrom is Roy without the hilt.
I have no problem with Ness and Lucas or Fox and Falco because they have enough difference from each other to make them unique. However I do have the same problem with Pit and Dark Pit as Dark Pit again offers nothing unique.
While I do agree on principle, Smash is far from the only fighting game that has similar or even clone characters. Or have them play slightly differently, thus getting very different reactions from pros or competitive players but not from casuals. Ryu and Ken are prime examples in their own series before SF3ish at least. Or Mortal Kombat's seemingly endless stream of ninjas.
Also, there's the competitive aspect of "Slight differences = big differences in playstyle". Of course, not going to affect how you play Smash that much, but I think it's worth keeping in mind when playing online. For example,
mains are often looking to dance, space and land those tippers.
aims to instead keep a solid pressure game and remind you that she's always ready to go aggro on you.
not only wants to keep you close and comfy for those hugs of doom - but also has ridicolous follow up damage.
might be the most gimpable of the bunch, but he'll want you on the edge. He kills the fastest out of the four. And not just those lulzy Aether kills either.
I know, it's not going to magically solve the "samey characters" problem, but it can be worth keeping an eye out for.