I never liked Mercs in RE5, but I think it's more to do with the characters and locations rather than the gameplay (I loved RE4 Mercs, which is the same minus melee kills giving you bonus time).
The faster pace of RE6 is better suited to a mode like Mercs if you ask me. Previous Mercs games were far less cheap, but whatever. All RE6 Mercs needs now is a mode without a timer, some extra characters like RE5 Reunion had (Claire, Jill, Barry, etc.), and possibly some more maps.
I personally think this Mercenaries blows compared to RE5. There scoring system is lousy, the game revolves around just countering and CDG. Maps in general are boring. Having a huge artificial difficulty spike towards the end is irritating. Nothing is more awesome than having a good run ruined because of one mistake.
RE5 is repetitive, but characters and strategy differed based on level. There was also a nicer progression in score as you got better.