Less Hair:
Seats needed to transport crews must and can be planned ahead and reserved. Not just cleared whenever needed kicking off paying customers.
Indeed they are for pre-scheduled deadhead. But, there are other times where it all happens quite quickly. For example, a flight experiences a serious mechanical condition at the gate or perhaps on taxi out. The flight is cancelled. But, the crew of the canceled flight now needs to be deadheaded with little notice to protect a downstream flight the same day. Some crew members might end up on the jump seat in such circumstances. But, some need to be placed somewhere in the cabin. It becomes an issue when the deadheading flight is full. But, in my experience it was always handled prior to boarding passengers.