Virtual users or system users for a mail server

Everybody seems to be configuring their mail server using virtual users
instead of the traditional system user approach.

I find system users easy to configure and even more secure, since IMAP and
POP3 processes can run under the system user account (of course disabling
shell access).

I keep wondering if perhaps I'm missing some interesting advantage of using
virtual user accounts?

Anyone like to comment on this?

-- 
Marc