To my amazement, Frontier seems to have had some success selling their empty middle row seats. I've seen them taken before the regular "stretch" seats. I guess this has given them a desire to go after the premium business.
BTW, can't you just buy an extra middle seat for yourself at regular prices? Also to my amazement, they've brought back $19 fares on 1000 mile flights. Couldn't I just pay an extra $19 for the middle seat to be free?