"No harm" except for the several slots that were taken up by people hoping to make a quick buck (selling the trade) who drop a day or two into classes and cause the students who actually NEED the class to be very stressed and annoyed and possibly have to adjust their plans because they don't think they will be able to take a class even though it will actually be free by the third time the class meets.

It also just fucks with the University's ability to gauge class interest. In my university, if a class filled up early in the registration window, the University would try to increase capacity or add another copy of the class, but that's not always an option.

A reminder that this is not done for technical reasons. Plenty of colleges all across the US, big and small, custom-built registration software or purchased on the open market, have fully functional waitlist features. "first come first serve" is a policy choice.