You know, this pisses me off for a number of reasons. The first, and largest, as that these companies need to learn that not every freaking game needs some kind of co-op of multiplayer mode! Mass Effect, in my opinion, is one of the best examples of games that was perfect with just single player. So for them to go ahead and shit all over that by adding multiplayer? It sets a bad example for other developers who may throw in similar modes that actually do poorly affect the rest of the game. Plus, I'm doubtful of how well this will work anyways. Having two separate studios working on different parts of the game often makes the entire experience feel at least somewhat disjointed.
And for those who say 'oh, the multiplayer is completely optional,' from the sounds of things it likely isn't. It sounds like the multiplayer mode will have it's own story and characters. That isn't a bad thing, mind you, but it also means I, a single player gamer, am cut off from the rest of the content. That could be fixed if they allowed for AI teammates, but that barely helps. In fact, in most cases, forcing AI teammates into the mix makes things much, much worse (*angry glare at Resident Evil 5*). But having no teammates, depending on how they handle things, could make actually playing the game next to impossible due to number of enemies and enemies straight. That depends on how they do with balancing though. And I know 'I could always turn down the difficulty,' but I always play through a game on the hardest difficulty first. I'm not about to change that for one game. And why do I do that? Because I can't stand not getting the achievement for beating it on the hardest difficulty. And I hate myself for that.
TL;DR: Not every game needs to be multiplayer in some way. Mass Effect, which is one of the best examples of SP gaming in my opinion, was one of the games to help prove that. Now that it has multiplayer, other games may force multiplayer in that will likely have poor quality and effect the SP mode. And as for the 'optional' argument, it isn't optional if you want to experience the full story and game from the sounds of it.