I'm guessing they went with Ike because he's popular, unique (enough so at least compared to Marth), and would advertise Radiant Dawn as well, even if he was the version from Path of Radiance. I always liked Ike myself, he's really cute. Not as big a fan of him in RD though, PoR is my favorite (especially Lord Ike, costume is nice). I have to wonder what was going through the artists head with the Awakening DLC Ike though. And to wrap up Ike-related thoughts, I'm pretty sure we'll see him in Smash 4, I don't see any reason to cut him.
And I'd love to see a more tactical Robin, but as amusing as Robin summoning Fire Emblem classes would be, it's probably too much for the game to handle. Though personally I think that summoning units to fight would also be a good way for a Nintendo Wars rep to fight (a series that, though I haven't played, would like to see in Smash). Random aside aside, I'd actually kind of like to see Robin hit someone with a book too. Silly, I suppose, but would amuse me. Maybe as a taunt or something.