It's very common to find mailboxes without a size limit during a Risk Assessment for Messaging's environments using Exchange Server.
In general such situation can put your Mailboxes' storage Capacity Planning under risk. Why? Because you cannot control what you do not measure. Do you agree?
Thus it's a very good practice regularly verify how many mailboxes does not have a size limit set or a customized one that is not under a specific policy. You can keep this documented and take proactive actions before something not planned takes place.
Attached to this post there is a Powershell script that helps to locate all mailboxes whose mailbox size limit is not set to use the Database Size Limit.
Remember: This script is not officially supported by Microsoft, use it at your own risk. Microsoft has no liability, obligations, warranty, or responsibility regarding any result produced by use of this file.
Regards and see you soon,