In a recent report by research firm Electronic Entertainment Design and Research, it was concluded that games with an online component sell nearly twice as many copies, on average, than their onlineless counterparts. You ever wonder why every game, no matter how unsuited, seems to be sticking in a multiplayer component? Now you know why.
Of course, there could be confounding correlations at work here, like between a game's budget and its propensity to have online play, or taking the possibility a step further, a game's level of marketing and said budget. Still, it's an interesting theory. Has the promise of multiplayer ever tipped the scales for a game you almost didn't purchase?