When you are out of ActiveSync licenses, you still have the ability to check the checkbox to allow ActiveSync for a user. You will NOT receive any error about the limit being reached, it just acts as "normal" with no message at all, and redirects to the user overview.
The suggestion is to disable the checkbox if out of licenses or raise an error when saving.
The current situation lets you think everything is alright, but the ActiveSync setting has not been saved (which you have to find out yourself, when you notice it doesn't work)
Also a reminder to the admin about the reached license limit would be great.
(Can those reminders also be mailed to a admin defined e-mail address?)