Why Did The Yankees Empty Their Farm System?

I understand that the Yanks got some good players in return for the prospects they gave away, but am I the only one who thinks the Yankees gave away way too much for the players they got this offseason? First they trade away Austin Jackson, their top prospect and one of the best prospects in all baseball with Phil Coke who I don't really care for, but have to admit, he was coming along. Then not to much after that they trade away possibly their top pitching prospects in Michael Dunn and Arodys Viscaino, I mean, now their basically left with Jesus Montero and that's it. I'm a big Yanks fan and will always be, but, why are they trading away so much young talent?