You kinda need it in some games, but not at the expense of single player. Seriously, developers, 5 hours campaign is NOT a good game when you pour at least 10 times that into multiplayer.
And another thing, some games SHOULD NOT have mulriplayer. BioShock 2, for instance. And I hear they are putting multiplayer on Dead Space 2. Yes, I'll still buy that one, but woe to EA if the cut corners in single player to make room for it.