That is something that has angered me in the past many times, and still often does. I love coop-modes, they´re usually great fun. My favorite coop-games were Armorines: Project S.W.A.R.M. and Turok: Rage Wars. But there´s a limit to how much you can give such games credit.
The worst offender in that regard was Resident Evil 5. Of course, a Resident Evil-game gets its hype, it´s too big of a franchise to be ignored. But when it was finally released, no, when its demo was released, it was really bad. Bad compared to Resident Evil 5. The controls are mainly the same as RE4´s, but somehow Capcom managed to make them feel off in that game. It also didn´t help that RE5 consisted of pure action only, no moody setpieces as those being found in RE4. What´s worst, though, is that when finally everyone dared to express their disappointment about the game, some people started to use the “but it´s great in coop-mode!”-argument.
Well, let me tell you this: Every game in video game history is more fun in coop-mode. Every single one. I´m sure, had Superman 64 featured coop, it would have been fun. That´s because games are always more fun when you´re playing with friends. It´s like watching a bad movie. Watching it alone just bores you, maybe makes you angry at the crap you´re watching. Now invite a friend, and the crappy movie turns into a funny movie, with you and your friend loudly laughing and talking about it while watching it. That doesn´t make the movie good, though. And you wouldn´t advise other people to watch it. It´s like that with video games. Together, you can laugh about it, alone, it´s just plain bad. That´s why it´s unnecessary to say praise otherwise not-so-good games for their coop-mode. It´s not something special.