I am an enormous fan of online co-op. That being said though, I think that co-op in a survival horror game would take away from the mood. Survival horror works well as a single player experience because it gives you that feeling of being alone and vulnerable.

I played through Resident Evil 5 with a friend using online co-op, and it was fun (fun enough to do it twice in fact), but it never felt like a survival horror to me. It became more like a 3rd person shooter with monsters.

I loved the mood and atmosphere in Dead Space. I would not want to see that compromised for the sake of online co-op.

That is just my opinion on the matter.