That's just my own personal experience but if've come to favour a reinstall over an upgrade everytime. I have upgraded both server and desktop editions of ubuntu at least tree times each and never was I perfectly satisfied with the result. Something always runs buggy, some configuration not migrated etc. A reinstall is worth the extra time.
Maybe the reason is, that the developer team can not test every case an bear in mind all the changes, you might have made to your system. Also some feature are never enabled in upgrades to not break the stuff users are used to.
A reinstall just results in a cleaner neater system all the components are designed to work with eachother.