Once upon a time, some airlines were proactive in identifying flight crew from their own or anyone else's airline and seating them in the emergency exits, cheapest insurance ever.
Then someone identified the emergency exit as an extra source of revenue and started selling off these prime seats. The seat pitch also reduced about this time, making the extra leg room even more desirable.
If the exit is not sold then there exists a chance for some cheap insurance by placing off duty crew in them or the opportunity to win some good will from the 6'6" guy or the frequent flyer by offering them the empty seat but to leave it empty is a missed opportunity either way.
It is, off course, up to the individual airline to ensure that their front line staff know, be directed and be allowed to take the initiative.