They got lucky last year in the postseason with the three man rotation. AJ was awful in his one start on three days rest, and Pettitte admitted (and it was clearly visible) that he had nothing in game six.
Now, this year, they eliminated the off day between games four and five of the League Championship Series. So unlike last year, when Burnett and Pettitte only had to pitch on three days rest in the World Seres, they would now have to do it in both rounds. And, on top of all that, the Yankees won't have the luxury of choosing the longer Division Series this year, so their pitchers would also be pitching on short rest in the first round.
It's a lot easier said than done to go with another three man rotation in this year's postseason, if the Yankees make it.