It's one thing to have persistent, growing characters (which they still are, despite starting on say, lvl 10), but what's the point of joining a game midway through and beginning on lvl 1?
If you want to start from scratch, join a new game, not one 8 hours in.
On the other hand, say every match/session/whatever you join in multiplayer ends at lvl 5, you just keep repeating the same first few hours of the game. This way you can pick up from where you left off with other people and not be penalized for it.