The Yankees don't have nearly as many impact bats in the minors, as they do impact arms.
First of all, how many "impact" arms hold up in MLB? "Not many"? Exactly. Having a lot of pitching depth in minor league baseball means nearly nothing to whether the team will have pitching depth at the MLB level in the future.
By the way, the Yankees need to fill 5 starter spots, and 7 spots in the bullpen. So although the Yankees have five pitchers in AAA that could possibly help the big club this season, in 3 or so years that current depth may not be there.
Moving on, how much more offense do they need? Here's what the Yankee offense could look like in the future -
C - Gary Sanchez
1B - Mark Teixeira
2B - Robinson Cano (I see no way he's not re-signed/extended by the Yankees)
3B - Dante Bichette Jr (I'm not 100% sure about Dante, but it's a good possibility)
SS - Derek Jeter (I'm fairly certain 2014 is his last year, especially as a regular)
LF - Brett Gardner
CF - Curtis Granderson (not as sure of his return as Cano, but either him or Swisher will be back)
RF - Mason Williams
DH - Alex Rodriguez
So the only real question mark on the offensive side of things in the future is SS, and Jesus Montero was not going to fill that spot anyway. And since the Yankees were not going to get a great pitcher to combine with Sabathia atop the rotation, without including Montero, then what is the problem?