Traveling together can be tied to their profile system. If on player can't show, they can stick him in camp, choose to play a different session, or perhaps control of his character can be delegated to another during this time.
Multiplayer should be a plus to the game that adds more choice...not a game killer.
Now I know this is the age of mobile gaming and networking, so perhaps a mobile / multiplayer version that is "less messy", and the single player version that is the "authentic experience".
The gen x-ers will probably leave the game running on the phone, that is sitting on the coach; abandoned at the first distraction that sends them running into the next room.
They will probably love the game too, and will talk about it for .687 minutes before the next distraction.
