Personally I love the Artist Top Tracks feature in Media Monkey where you can write the Last.fm popularity into a predefined tag. This is excellent for selecting songs for playlists.
Well, you can make a custom tag of it in Musicbee, just look how the field is represented under tag inspector.
I have a lot of tags I no longer need. I am in the process of removing all replay gain-tags and redoing them, now collection-wise not album-wise.
BTW will removing tags remove the tags themselves or only the values?
Replay gain is usually included by the encoder based on a reference sound threshold. Unless you set it to something other than default, you are just going to get the same values. As for removing such tags, make a autoplaylist where the replay gain field isn't empty, select all, SHIFT+ENTER and delete the tag. For myself, I would likely export the autoplaylist to a m3u playlist and open the m3u in mp3tag. Also, in my experience, deleting a single tag field under tag inspector wiped out all of the tags (I never got around to report it).