Here's my take.
I had a friend for over a year. He was always trying to get me into Magic, DKR, and SMB multiplayer stuff, and I utterly resisted. The last co-op game experience I liked was FFVII with my brother, and that was 15 years ago. Now, multiplayer can **** bricks. I don't need other people to muck up my gaming experience. Adding co-op gameplay to what should be single player campaigns just strikes me as straddling the fence. You can't decide what will sell, so you **** it all up by being mediocre. Look at Catherine; it's purely single player, with no inkling of need or want from multiplayer nuts, and I'm more excited for that than any MAG, CoD, or whatever stupid shooter they make so online nerds can earn kills and spam grenades.
Hell, the attempt they made at co-op with DQIX was fun...for five minutes. Purely single player campaigns should be able to standalone. Pick a market for your game and stand by that decision.