I mean, yeah, if you consider rebounding and running a team "roles." By the same logic you could consider Ben Wallace a role player, but that doesn't mean he wasn't a pivotal part of their championship (and an All-Star). No, Rondo, probably can't be the best player on a championship team, but I don't see anybody ever claiming that. If he had 3-4 other players on the same tier as him like Big Ben had, I don't think he'd have trouble winning a title whatsoever.