Opportunity cost? It isn't that hard to figure out why someone would want to pay for something they don't have to do on their own. You can get cleaved while you are at work/sleeping, come back and then do other stuff instead of leveling during your actual playtime.
You missed my point that while not every party will let you do this, it's completely possible to do what you describe in an ordinary abyssea party (especially if you're friends with someone in the party)