cyrus-sasl is an authentication library used both by mail servers and mail clients. Installing it does not rely on the existence of any particular user (as mailbase would ensure is set up), and it doesn't make sense to add the mail and postmaster users when a machine is set up only with a mail client. If there is a package which depends on dev-libs/cyrus-sasl which assumes the existence of the mail user accounts or the existence of /var/spool/mail or similar, let that package depend on mailbase instead of forcing it here where it is sometimes not needed.
If there is a reason for dev-libs/cyrus-sasl to know or care about the mail accounts, use a USE flag so that can be disabled.
I'm interested in enabling SASL support for ZNC. Why do I need mailbase for that?