Actually I can use that against him. You see, if a character does not bare interest, things are not exactly going to workout. As you said, others are not exactly fond of the idea of Chrom, which hurts more then helps in the long run. A vocal minority stating otherwise won't exactly save the character for long. If the Super Smash Bros. fanbase is hardly trilled by the character, imagine how the larger casual crowd will think of the character. Probably general sighs. The problem with Chrom is that the archetype has been done already, and people would rather look for something more fresh. While yes the character could be included, there wouldn't exactly be a lot of praise.
That isn't the point. The fan base is split. If the entire fan base didn't like Chrom, it would be different. But that isn't the case here. At this point, Roy and Chrom are the front runners for representation.
If we are going to talk about the appearance to the larger audience, then Lucina would be hit even harder by that. She is largely (LARGELY) based off of Marth. So if she gets into the game, people would say/think "A female Marth?". Not to mention their play style could be incredibly similar.
I'm sure there are several characters that some people are not fond of because they don't find them interesting. What makes a character interesting in Smash is their style of play. Not what they act like in their own games.
Your argument that the archetype has been done already can be applied to several characters if we are going by swordsmen standards. Link, Toon Link, Marth, Ike, Roy and Meta Knight all use swords, but all have different play styles. Really, if Chrom were to be added, sure, he could be a clone, but there is just as much a chance that he
wouldn't be.
One other comment that is not really a response, but I don't get the clone argument against Chrom as far as defending Roy. Roy WAS a clone when he appeared in Smash.