You could say "what's the catch" to a lot of moves and characters in SSB, it depends on how you make the character play, and I'm not saying copy and paste from the pre-existing PSAs, you can do a lot of stuff with all his chaos powers, you can use your imagination too, I'm just saying he brings a lot of material to the table to put to unique use
I think he's somewhat iconic now, not nearly as much but damn if I've ever met a person who hasn't played Sonic Adventure 2 and loved it(except one of my friends), he was of enough importance to Sakurai to make him an Assist Trophy
And another big point was to make it easier for the PMBR:
Shadow is the same basic skeleton as Sonic, and there's already a crap ton of animations for Shadow out there that could just be tweaked if it was even needed, honestly this should be enough of a reason right here, not even considering the fact of his popularity, you have to think on what's a reasonable thing to ask from them, making a character from scratch is too much
I quit Sonic after SA2, Heroes was crap and the rest looked worse and worse
I'm just saying I think the PMBR would get a lot less flak from hardcore Nintendo fanboys and SSB enthusiasts if they made AT's characters since they were already somewhat in the game, this is all assuming they'll even add someone in besides Roy, but I don't think it's bad to discuss, they HAD to know this would happen when they revealed Roy
I don't wanna argue about this anymore though, it's up to the PMBR, but it would be awesome to see some polls if they do decide to add another character