Correct me if I am wrong, but Frontier isn't in the same place as Spirit by any means.
I mean to say, Frontier is growing to try to stay alive. Spirit is growing cause they are making money. Plus, starting a fare war with an airline that has the lowest CASM in the industry when you aren't making very much money may not be the best idea.
I just don't see Frontier being able to really pose much of a threat to the Spirit business model.
Then again, what the heck do I know.