Not everyone has played a Fire Emblem game, and as a result, many simply wouldn't care if Robin got in. As col as it would be for Robin to get in, they would just likely give him/her just two appearances, male or female, based on the default, and use the same move set and just have different voice packs. Basically what their doing with Wii Fit Trainer, and possibly the Villager.
Where as with Mii's players would have full control over what they look like, and possibly have a verity of voice types to chose from.
No one practically heard of Fire emblem when Melee came out.
And you're right. People do love to customize their characters.
If they allowed for full Robin customization, people would likely go crazy over that, buy smash. Customize their characters to their liking, AND probably go buy fire emblem as well.
Why would they give full voices for Mii's instead of Robin?
There's much more potential in Robin than Mii.
I forget though, this is Nintendo's market....
People who claim to hate the idea now will eventually get over it while using their Mii, if only because there is a level of satisfaction one can only achieve when using a character that represents one self
You underestimate our hatred.
Certain things just don't go together for some people.
This is like throwing a Digimon into Pokemon
This is like throwing Kratos (god of war) into Mario
This is like throwing cars into Banjo and Kazooie
This is like throwing Oil into water.
These things just don't mix.