With regards to allocating ABP's in to exit rows, the main problem is the travel agent selling extra legroom seats to 80 year old Betty and her husband 81 year old Percy because betty has just had her hip replaced and cant bend her leg. They they turn up to the airport to check-in and are told that they cant have these seats because of
A. Their age
and
B. A medical problem
You also have to bear in mind that the airlines (especially a large one with blue aircraft) will not allow the movement of pre-alocated seat. So you call their office and explain and they tell you to give them the seats as they have paid for them and let the crew sort it out and give them a re-fund.
As a dispatcher I am the first target for the cabin crew when they find out that a non ABP is sat in an exit seat and wont listen when you tell them that it was an instuction from their airline to leave them there.
I dont think that 90% of people who sit in an exit row are fully aware of their responsibility, however charging for these seats allows the checkin staff to decide who sits there and usually eliminates some of the rif raf who are not responsible enough to sit there.