S-Ranks dont get you much if you've already gotten the character's costumes and the stages. Only really what Saiyaman said, titles for your dog tags, they could be used as unlock requirements for future characters and/or stages however.
It's B rank to get charictars (Or was it B to get costumes an A to get Charicters I don't rember), most likely that's just the first time they got over that on the map that unlocks Sherry, for Mercs. Could also branch out to the other modes as well I guess when they're released.
Self Proclaimed Offical HoonDing, The Yokudan God of Make Way of the TES:V Skyrim Boards (360/PS3)