I realize this is an older topic but the virtusers table can be used in a more useful way than just aliases.
Currently, the virtuser table is the only way you can integrate roundcube in with a virtual user email address setup. It'll pull out the username from the database 'ie,
foo@bar.org' and send it off to the email systems authentication system along with the password provided.
Its the only way to get roundcube to work for those of us who have setup so-called 'isp-style' email systems!
Very simple but powerful features I'd miss if it wasn't there
-- Les