If for whatever reason you really want to see all the different dialects in a single search instead of looking at them individually, this can already be accomplished by searching for ~hiroshimaben ~kansaiben ~edoben, etc. We already have tag_group:japanese_dialects to help with finding the individual tags, so I don't see the point of creating yet another umbrella tag just because we can. Tags exist for search, not to organize everything into hierarchies.
As for "less popular dialects that don't get their own tags", burying them in a catchall tag where they will get lost amongst the other posts doesn't solve anything. If they're worth tagging at all, they deserve their own tags.