Whether or not Megabox itself is a good thing, there's every reason to want to use a middleman when distributing music. Do you enjoy setting up web servers, employing web designers, handling CC transactions, selling ad space, etc? Or do you really enjoy making music and don't mind giving away a 10% slice to have someone else do all that?
I run a small music web server as a favour to a friend[1], and honestly it's a pain in the neck to manage it. If we could hand the whole lot over to whatever is the next MySpace, then we would.