Yes, I have noticed that, in Early June I flew AA PHL-LAS for $178RT, it was a match of a Spirit fare. For awhile they ignored ULCCs but I think with oil being so low, they figure this is a good time to run them out.

Now about the fares, I see the $15 fares but they are only valid on most routes on Tuesdays and Wednesdays. There are going to be people who won't be able to travel both to and from their destination on those days. In the PHL-IAH market for SEP/OCT the other days are pricing out at around $59. But even $59 is still a bit low.