I really don't like the idea of either Starfleet Marines or an unified Federation Army, both seem too militaristic.
Technology has rendered large scale ground combats mostly irrelevant anyway, and for small assaults Starfleet has their security teams.
Well, there are plenty enemies of the Federation that have large ground based armies, the Klingons, the Jem'Hadar, even the Cardassians have enaged in a good many ground battles.
Hmm, for that matter, Chief O'Brien has an unpsecified background as a "soldier." We know he was in many ground fights against the Cardassians, and as of DS9's second season him and Major Kira were the only ones on the station considered to have any real combat experience. O'Brien's obviously not a Marine, so maybe Starfleet does have a ground division that exists as part of its rank structure?