This is a great question. With online reservations of exit seats for elites, the seat assignment is typically not done by the GA any longer. I've seen people I'd consider unfit for emergency exit duties seated in these rows on many flights; especially the elderly and obese seem to be highly attracted to these seats. FAs rarely seem to care. I've never seen anyone moved out of an exit row (hey, who would say "I'm not confortable sitting here, I think I might be too fat"). Btw., I love the "you can't sit here if you need a seatbelt extension because it might hinder evacuation" explanation on the safety card!