Right now, MB allows to recover genres from Last.fm with the new tags overwriting the Genre tag, this way:
It allows too to recover keywords from Last.fm, overwriting Keywords tag:
The genre tag cloud in Last.fm is much more useful than the keywords tag. But in case you already have a organized library, recovering those tags means overwriting all your organization.
Keywords stuff works extremelly well when using the Library Explorer. So, why not giving the chance of recovering the X more popular tags (top screenshot) and to overwrite with them Keywords tag INSTEAD of Genre tag.
This way, you still keep the way you organized your library, but you can use the last.fm genre tag cloud through Keywords tag and Library Explorer.