Out of all these threads I haven't seen one yet responding to the talk of multiplayer modes in games that before didn't have them. I think the problem is two-fold.
First, if you're going to do multiplayer, the only way it's going to be of any value to me is if it actually manages to replace Call of Duty, Battlefield, and Team Fortress on my roster. I just don't have the time for more than a handful of multiplayer games. You can either do that by out-doing COD and Battlefield - the chances of which for most developers are microscopic this far into a console generation, or doing something so innovative that it inherently stands out.
Other multiplayer modes have basically been standard shooter multiplayer with quirks, but the only two I've played that really managed to stick out on their own are Left 4 Dead and the old Splinter Cell multiplayer. Those still had some of the fundamental FPS interface, but their rules and objectives turned them into new kinds of multiplayer games entirely.
Secondly, the reason they do this is to increase replayability, and multiplayer isn't the only way to do this, as we're now discovering with the new trend: horde modes. I've actually started to prefer Firefight mode in Halo over multiplayer just because I don't wanna have to deal competition that I can never match, yet Firefight is just as replayable. That's just one example, more games might be worth it if they could create endlessly replayable, almost mini-game-like modes.