I fail to see the need for every game to have some form of online and/or multi-player capability. Some games just work better as single player, Fallout 3 being one of them.
Did it really need co-op? No, not in my opinion. Will Vegas be lacking due its lack of co-op or multi-player? I very much doubt it, I'm just grateful they didn't try and shoe horn something in just to be fashionable.