If I may weigh in here, at the risk of offending the gravitationally-challenged, I think this idea is way overdue. The issue is particularly noticeable when making the transition between my usual stamping ground of Asia, where the majority of travellers are of diminutive stature, to the USA, where they are not. I don't think we are doing anyone any favours by ignoring this matter, least of all those in need of an extra seat. I have a friend, who is of the "pie-shifting persuasion" and he positively dreads the prospect of flying in the smaller-cabined aircraft, such as ATRs. I am sure if the option to purchase an additional seat at a discount were available, he would not be offended and would jump at the chance (if he were not too fat to do so).