Assume you have a 50 user license and 40 active users.

When you import a CSV file with 11 users...

• You have no chance to get the 90%-warning-email in time
• Everything works fine, until Crowd recalculates the users again (which may take hours)
• After that, you're pretty much screwed since no application is able to authenticate its users any longer.

We've been screwed two times so far.

I think a warning during an import, that one's going to exceed the license limit with such an import, would by a very quick win.