I have also noticed that the roaming cache is large (2.9GB and >9k files). Most of this (2GB) is "ArtistBackdrops". I'm assuming that these could be safely deleted and would just re-load as necessary.
As I only use the desktop (windows) vesrion of MB (build 3.1.6388) is there anyway that this could be made into a self clearing cache that downloads the backdrops during each session and then deletes them when the session is closed?