Just a heads-up... found out that although the Frontier website does not show any flights for 2016, Frontier flights can actually be booked through the Apple Vacations website through March 2016 (at least):

http://www.applevacations.com/
(Select "Flight Only")

The downside is that their prices are only marginally better than American. PHL-MBJ is still close to the $600 point. Might be worth to keep an eye out to see if they ever adjust the prices to the were there are for the fall.