Scenario: for the occasional conference/event, it's handy popping up small MX and a couple of APs. What's not handy? Paying for licenses for gear that sits on the shelf until needed. Would be nice if an X year license decremented on actual days used in this case somehow.
What bothers me is that if you purchase units and they sit for a month or two before deployment - before the license is even activated - the license countdown has already started. In fact, the license countdown starts the day the license is issued. Drives me nuts, but I try to ignore it.