It was "bought" because the improvement that lead directly to the WS championship was provided by the big 3.
Exactly.
This Yankees team wasn't even in the playoffs last year. They go out and buy Sabathia, AJ Burnett and Teixeira in the offseason and BOOM, World Series!
Now the 2007 Red Sox were coming off a playoff-less year, but you could argue that Lugo, Drew, Matsuzaka were not what brought them back to glory. 2 of those 3 were nice additions, but it was about the rebound of Josh Beckett.
Beckett, acquired via trade and embarassing in his first season with Boston, drove the Red Sox 2007 postseason. (Along with some good contributions from ROY winner Pedroia, solid first baseman Youkilis, Papelbon, and getting late spark from Crisp being benched in favor of an emerging Ellsbury.)