Gregg,
I can't speak for the P3, but on the P2, the Atti. mode has 2 real advantages. Firstly, it uses no GPS, so when flying indoors, such as in a Gym, with no GPS reception, keeping it in GPS mode could cause the Phantom to drift more than in Atti. mode where you have all control. Secondly, in Atti. mode, the Phantom will fly about 10-15 mph faster.
I rarely use atti, but when i'm in the park flying, I will switch to it for a few minutes to remind myself how to fly with it incase I am ever in a situation where its needed.