Isn't one of the reasons for the drop-in/drop-out feature the fact that the official campaign is that long? Jumping into other games with different completion levels is what will happen when you jump in and out of your friends' games.
A list of hosted games (with details mentioned in previous post) would just help you pick a session you would like to join out of all the public ones. I know I would want to browse through all publically available games when none of my friends are available.
Being able to browse would also let me see if there's someone else playing a user-made campaign that I have or would want to try. How else would one join a session with a custom campaign?
