In addition, there may be feature-request posts that are about questions. If these were to be tagged asking-questions instead, then it would have a different connotation. The latter really implies more of a support request.
Admittedly there's a bit of a gray area (and of course there's nothing wrong with having both tags on the same question) but there are clearly cases when a meta question should have one tag and not the other.
At the end of the day I'm inclined to follow MSO's example; they haven't merged those tags despite quite a bit of overlap and high volume.