I think they don't go so quickly as the price set is quite high. US$100 per long haul sector adds $HK 1600 per round trip which is a significant price on discount tickets.
It is not the USD100. If is free for MPC SL and above. As Cathay Boy said. the arm rest cannot move. In addition, many extra legroom seats are next to the lav (that is why I only like the front row for 77A, which is not next to a lav). I would rather take a seat with an empty seat next to me.