You're forgetting Stabbey, that sometimes the host simply cannot play the game for quite some time, and he wouldn't mind you (guest) continue the game with someone else.
It might be interesting to have savegames labeled with a 'time played in coop / total time' tag. If you want to join a game in multiplayer you'd rather have one where both characters are ~equally strong; that might not be the case when the savegame was made exclusively in singleplayer, thus overpowering one character in favor of the other. A high percentage in this tag means it's a good game to join for multiplayer.
