Does the stick to a game suggestion really hold?

At the moment I have a decent roll on Pokerstars, Fulltilt, and Bodog. I find pokerstars to have the best tournaments, bodog to have soft cash games, and full tilt has ironman meaning I can put in a lot of volume in SnGs to qualify. I'm a decent tournament player and I'm getting better at SnGs, they're semi similar game types, but cash is pretty different. I don't have a lot of experience at it, but wouldn't mind getting better. If I start playing multiple game types am I risking losing focus on one? I have heard a lot here that you should pick a game and stick to it. Are cash and tournaments/sngs different enough that I risk poisoning my play to try to do both?

I might be missing the point, this advice may be directed at people who are just looking to get better and eventually be profitable. I know quite a few high stakes players are proficient if not great at a variety of game formats and types, but with limited time for study would I just be better focusing on one format?