So what happens when someone moves to an empty exit row seat after take off? Do the flt attendants stop their face painting or whatever, race up and say that'll be 50 bucks thanks!
What happens when the flt is cancelled and the pax are put on QF? Does the punter say I'll have an exit row seat thanks cause I've paid extra for it!
Imho it will cause more problems for cabin/ground staff than its worth.