That is a dangerous game to be playing. This head to head bull crap is what got this industry in trouble after deregulation. Would Spirit management be stupid enough to play this game? Absolutely. However...
If anything, I think Spirit and Frontier may be positioning themselves that when/if they do merge they are creating overlap (such as in ATL) to play the DOJ merger approval game. They are in a sense creating overlap so when they merge, it appears they are 'working with' the DOJ if one of them brings down a route out of ATL rather than give up a more lucrative one.
Not sure how big that void is. Say one wants to go from BUF to TPA now. Before, with AirTran, if they didn't get a nonstop from BUF to MCO, they would go through ATL creating an artificial bolster of that hub. Now, they are looking to go from BUF to MCO and they have to connect, that traffic is still there, but being brought through BWI.