The big advantage is app developer control. Imagine being able to update your customers/users
application without being dependent on N different distributions somewhat random selection of
software, it also avoids "Oh to get our next update, you need to update your distro" which is
completely unacceptible from an administration point of view.
Distro's tend to want monolithic control over everything, even if it potentially hurts users and
developers. The problem with security updates wouldn't be the distro's responsibility if they
didn't have control every bit of software on your computer.
In practice it's similar to iphone/appstore, but they are evil and distro's are heroes.... ;)
Yes i know about principal freedom, but time is also a cost...