I can definitely see an advantage to their approach.
there are parts of a linux distro that tie in with lots of other things in the distro and are rather hard to upgrade independently (things like KDE, GNOME)
on the other hand, there are a lot of things that really can be upgraded independently (firefox, openoffice, games)
having a distro that split between the periodic update and the rolling update models, doing periodic updates for the things that have lots of interdependencies and rolling updates for the things that don't could be a very useful variation.
even better would be if the user could specify which packages get modified in which manner (with suitable defaults).