EDIT: It has been my experience that doing a distro upgrade via the Update Manager will leave you with broken apps and will uninstall some of the things you have. A fresh install is the best way. Back up your stuff ie: your Home directory to another partition or if you have one another drive....
What about for people who aren't like us, who don't usually reinstall their systems? They're going to have a lot of stuff that they don't want to have to back up and put back together again. They'd just rather keep their OS "as is" and hit "upgrade" and have it work seamlessly.
It's these little things that are keeping Linux from becoming mainstream.