A couple of years ago I read a story that was lined on Slashdot about the impact of online modes on videogames ...
Games with online modes tended to sell better than those that didn't have online modes, but when adjusted for game quality (through review scores) games with online modes sold worst. The percentage of gamers who actually tried the online modes in games was (something like) 20%.
From personal experience I don't think that online gameplay is important to most gamers, but the gamers who are interested in online gameplay are REALLY interested in online gameplay.