Haven't seen a game yet with a co-op mode where I didn't have this nagging suspicion "I wonder what they short changed on single player to institute co-op?" And I can actually think of an open RPG I liked (Two Worlds) where they put out two addons available for multiplayer only with 90 quests. 90! And the enemies were toughened accordingly that you really couldn't manage them solo. It sucked, almost no one used multiplayer and it was a great waste of developer time and energy. A number of folks lamented the fact the single player game got shorted so badly.
Keep single player RPGs single.
But you are missing one point there... Two Worlds was released first on PC as a MMO. Did you want them to port this game to 360 without multiplayer? It wouldn´t make any sense.
The problem is that those devs had not the experience, money or time to make a decent port to 360 (not to mention that even on PC the game was not soo good, but it was way better than the 360 version).
