HRod's bat should play well but his somewhat limited homer power and his average but adequate defense at several positions just screams super sub to me. That's not a negative either - that's a really valuable player. Infield outfield plus he switch hits - lefty - righty, any kind of match up, any place on the field you need him except catcher and maybe center field. Yeah, he'd play almost as much as a regular.
Cozart and Gregorious - if DiDi's bat continues to develop I think I agree with Scrap - there's so much value in trade you about have to move one of them while that value is high. If Didi doesn't improve much as a hitter he's still a great sub for SS just as he is, though, it's a can't lose situation for the Reds.