I understood that the CAA were looking at this policy with some concern. The main reason wasn't so much the potential for unsuitable passengers to be seated in ABP rows, where I think there has to be some reliance on check-in staff and cabin crew to police this. Indeed, some tour operators selling extra legroom seats (Thomson is one, I think) state that if you pay for an exit seat but do not qualify through age or physical disposition, they won't refund you.
I had heard that the CAA's main concern was on less-than-full flights. If an airline is charging extra for legroom seats but there is no take-up, the seats adjacent to the exits are likely to remain empty as passengers are checked into other rows. In the event of an emergency on that flight, there is no able-bodied passenger sitting next to the door to be able to open it quickly and initiate the evacuation via that exit.
Crashdive is right, in that some airlines have deliberately not gone down this road. Long may it continue.