I think the need for having aliases for the same tag is best illustrated by the myriad ways to write "hip hop", "hip-hop" or "hiphop". Also see http://musicbrainz.org/search?query=hip+hop&type=tag&limit=100&method=direct

Entering one of these aliases should resolve to the tag's main name, whatever we'd decide for in that particular case.

Like with other entities, aliases should also be settable as primary for a locale. E.g. "classical music" would be "taidemusiikki" in Finnish.