I can think of two reasons: quality control and package support.
Debian on testing and sid repositories is really the wild west. Packages are put out there and many times it will break. When Linux Mint offered a Debian version, they had to move to Update Packs where the team locked down a repository, tested the heck out of it, and released through their update manager. Even then there are issues and maintaining that release has been a lot of work and the Update Packs are few and far in-between. The Ubuntu based versions of Mint are still more recommended.
Ubuntu essentially does the same thing with locking down packages before a release and will often use older versions to make sure everything works. There is a post by Jeremy Bicha about the Ubuntu team swapping out Gnome versions for performance and to ease performance issues. There is also recent news that Gnome 3.4 will make it into 12.04 and they are hard at work fixing bugs. It is good that they do that because with Debian, they are just put out there and maybe it works. Maybe it doesn't.
As for package support, not only is Ubuntu generally more supported with popular packages and mirrors serving them, but the PPAs at launchpad.net are the icing on the cake. I've tried pure Debian and LMDE and the quality of packages (support and latest versions) by 3rd parties is night and day.
Now don't get me wrong.. Debian Stable is considered rock solid even though the packages are old and it is supported for 2 years. Its just that Ubuntu LTS releases on the desktop and server have 5 years of support so you will be updating your OS less on Ubuntu than Debian.
The best part of using Ubuntu? You don't have to use the official versions. You can try any desktop available and derivatives like Trisquel with the rock solid Ubuntu foundation to build upon.
