I like multiplayer shooters. By a large margin, Call of Duty gets the most amount of spin on my 360. However, my first and true love will always be single player adventure/rpg games. I don't know if ME3 is going to be the end of the series, but I'm hoping it's not and if it isn't, I don't know if I completly support mutliplayer for Mass Effect.
Halo is a good example of why I hesitate to vote for multiplayer in Mass Effect. Halo has a very deep narrative. It was only really hinted at in the first game; just enough to wet the appetite. Thing was, the most popular feature of the game turned out to be the multiplayer LAN support. So when the sequel comes out, the narrative is fleshed out a great deal more, but then so was the multiplayer component. Again the multiplayer was the most popular part of the game. Then third game. The campaign, by then, was whittled down to an well polished afterthought. The multiplayer became the real show.
I have a real fear something like this will happen to Mass Effect if multiplayer is introduced. Bioware made a quantum leap in shooting mechanics between ME1 and ME2. I imagine similar progress will be made between ME2 and ME3. In terms of gameplay quality, that would put ME3's shooting mechanics in the relm of a mainstream online multiplayer shooter. I doubt Bioware will be pleased with just being in the relm, though. Likely this games shooting could could surpass that of other online shooters like Gears of war and Red Dead Redemption. If that is the case, people will understandably gravitate towards the multiplayer, and begin to ignore the campaign. There will be very little incentive in providing a campaign of the depth that the previous titles enjoyed (particularly in a game as deep as Mass Effect) for any future titles, if the title becomes ubiquitous with multiplayer shooting.
That's not necessarily a completely horrible thing. As I said, I really like shooters and I think a Mass Effect multiplayer shooter would be cool. In a perfect world, we could have the best of both. One Mass Effect title that is in the vein of the previous titles. Then another downloadable title that is strictly multiplayer. Maybe even allowing for importing a customized Shepard from the campaign to the multiplayer. And in the other direction, weapons unlocked in the multiplayer could be imported to the campaign. Synergy.
Of course this could all be packaged in the same box, and it would be a greater value to the consumer that way, too, but there it would inevitably lead to the previously stated 'Halo' scenario. There's only so much space on a disc. If mutliplayer were downloadable, the campaign would not be fighting for space.