The reason this is not directly supported is that it encourages people to regard tags as mutable entities that can be changed at will. This goes against the general principle that a tag, once cut, never changes.
Technically you can change a tag, but this is meant to be a measure of last resort not something you do as a matter of course.
That said, I understand the requirement - if you are using tags as markers for different segments of a private history, I can see it would be useful to move the markers when you rebase.
Perhaps there might be a case for a kind of marker reference that does move with a rebase, but it would be somewhat invasive to implement so you'd have to convince the list it was worthwhile.
jon.
Jon Seymour <jon.seymour@gmail.com>