No, the long campaign is not the reason for drop-in, drop-out co-op. Sure, you COULD do drop-in, drop out with random people, but the one who joins will always be stuck with whatever the host's world looks like.
You're always joining the host's game, and using their progress and accomplishments on previous quests. Sure, the joiner can save their progress if they want to continue, but they can't join another game with the things they've done, it's all dependent on the host.
There's no way a simple server browser could show you the progress on all the possible quests the host has done.
