I would like to see this feature too. I have a large non-shared collection and quite many of my songs have been rated mainly via MediaMonkey, which stores them into POPM tags. I'm often creating dynamic playlists based on tags, and the current support for that in subsonic is very limited, which is a shame as it would be nice to create playlists based on that data while on the go.
So, what we would need is that Subsonic would read that tag info on sync, store it into its own DB, and present the data in the web-GUI. Could be read-only on the first phase, adding support for 1) db updates, 2) tag updates, later. I very much would like to see the tag update option too, but it should probably be a user-specific permission setting. POPM does support multiple ratings from different users, but big sites probably don't like their files to be changed, while most private sites with 1 or 2 users would like that.
POPM tag is documented in
http://id3.org/id3v2.3.0