Figured I might as well be the first to bring this up...
Some relation to but not the same issue as: What ubuntu versions are covered?
At time of writing, there are currently nine instances of the "Ubuntu" tag on the site.
To me, this makes roughly as much sense as tagging a StackOverflow question with 'programming'.
I'm going to propose discouraging use of the "Ubuntu" tag. If a question is specifically tied to a particular variant, of course, it should be tagged with that variant (e.g. Kubuntu), but for "Ubuntu Prime" or general questions not tied to a particular variant, it's redundant.