Each passenger desires to fly comfortably. However, this is far from all. In economy class, the distance between the seats is not very large. And swivel chair in front a neighbor causes a lot of inconvenience.
Citizens who flew at least a few times in my life, believe that such extra seats and even more, if the distance between destinations is small.
Some believe that in the long-haul flights as necessary to ban such sites, but experts say that in 8-10 hours' flying time a person needs to relax.