What about passengers that move themselves? Often the airline will try to keep an empty seat next to the higher level preferred FF'ers, and of course this means an open seat in an exit row or bulkhead unless the flight is really full.

A recent flight was slowly filling up and our exit row middle seat was empty. Then someone took it and I didn't think anything of it. A little later another passenger came along and claimed the same seat. The person in the middle row turned a little red and muttered that his real seat was in the back of the plane and left.

That got me thinking: hey, maybe he's onto something here? It's not like I own the empty middle even though the airline tries to leave it open as a "bonus" for being a preferred FF'er. If I find myself with a really bogus seat (or bogus co-passenger) and see an empty middle seat in the exit row should I take it?