That wouldn't be entirely true. You could always setup something to monitor disk usage outside of cPanel such as Nagios, although I'm not certain why you'd want to monitor it (versus disk partition space usage).
You could also set for users to receive email alerts for email accounts in WHM > Tweak Settings with the following:
Enable mailbox usage warnings
Mailbox disk quota “warn” percentage
Mailbox disk quota “critical” percentage
Mailbox disk quota “full” percentage