Besides Left 4 Dead and possibly Halo 3 (since it's 4-player as well), I can't see any time where playing a game Co-op with a complete stranger is a good idea. You are completely in the dark about how open your partner is to taking suggestion and direction, how skilled they are at the game, and how seriously they take the game itself.
Take Resident Evil 5. You friend can't get to their Xbox this week. You want to knock out a few levels while he's gone, and you need a Sheva, so you get an annoying kid who picked up the game this morning and can't play worth shit. You end up failing miserably and your fun is ruined by the terrible partner picked for you. Or worse yet they go off gung-ho and die, leaving you to clean up alone.
Actually...now that I think about it, maybe Left 4 Dead should only be played with friends too. :\
Take Resident Evil 5. You friend can't get to their Xbox this week. You want to knock out a few levels while he's gone, and you need a Sheva, so you get an annoying kid who picked up the game this morning and can't play worth shit. You end up failing miserably and your fun is ruined by the terrible partner picked for you. Or worse yet they go off gung-ho and die, leaving you to clean up alone.
Actually...now that I think about it, maybe Left 4 Dead should only be played with friends too. :\