To be quite fair, even the sword wielders themselves are fairly unique from each other.
A warrior elf with multiple projectiles.
An elegant sword wielding prince.
A warrior possessing a fiery blade.
A mercenary that carries a two-handed blade with one hand.
A cartoonier version of the warrior elf.
Yes, which is why I generally don't complain about sword users, because it's more than just a sword.
Link and Marth are undeniably shoo-ins, and the amount of overlap between them is insignificant, like the overlap between Captain Falcon and Sheik perhaps.
Ike fulfills a perfectly fine niche, being the "big sword big smash" guy.
Toon Link honestly isn't much different from Link, but I think it being 50% of the LoZ Franchise is enough. There certainly should not be another "link" though. He is somewhat unique being the "boy swordsman," though, but whatever.
Roy, on the other hand.. The only thing that separates Roy from Marth is "I have some fire." If Roy was a mixture between a "Firebender" and a "swordsman prince," then it would be much better, but he's not, and it's basically just some very mild visual affects that change it.
Because of this, when there are people saying, "let's have Marth, Ike, Chrom, Roy, Anna, Lucina, Lyn, and Takamaru," then I have no clue what to say.
Marth, Anna/Lyn/
Takamaru, and Ike really seems like the only 3-character option that allows for divserity between the characters. This is only if Anna/Lyn/Takamaru include some non-sword attacks, like kicks. Anything beyond those three has significant overlap.
I could only see Chrom coming in if he took Ike's niche, but he doesn't fight for his friends! ;(