Outside of advertising and being on the box art, Chrom and Robin are pretty much equal in importance as protagonists (I put much more weight in what happens in the game than the advertising). Robin's the first character you see after starting a new game, the first battle is Chrom and Robin teaming up against the supposed main villain, and the two are absolutely inseparable throughout the entire game always being there for one another out when the other is struggling.
I also feel that Robin contrasts Ike and Marth beautifully and would serve as a god foil to both while maintaining how well Ike and Marth serve as foils to each other already while the things that make Chrom a great character in his own right and distinguish him from Ike and Marth (how outgoing and how informal he is, his hero complex, his straight man tendencies, etc.) won't transfer over to Smash very well if at all so you're stuck with the obvious similarities while diluting how well Marth and Ike contrast each other already.
..........though Ike, Marth, and Chrom in a 3 Musketeers pose would be sweet.