I disagree. Your argument relies on a flawed premise. If we accept your figure that 58% of their roster is "home grown" it does not logically follow that the talent on the team is distributed equally amongst all 25 players. Clearly, much of the big-name talent on the Yankees club has been bought or aquired via trade. This includes A-Rod, Teixeira, Damon, Sabathia, Swisher, Matsui, Burnett and yes, even Pettitte who obviously started with NY but was aquired via free agency after a stint in Houston.
Of course, I have always considered this argument a ridiculous one. A championship is a championship, I don't care how the players were aquired. I never understood why people got all defensive when being accused of "buying a championship". Just to use the Red Sox as a comparison, they have "bought" or aquired via free agency... Manny Ramirez, Beckett, Wakefield, Foulke, Pedro Martinez, Schilling, Timlin, Embree, Varitek, Lowe, Ortiz, Damon, Bellhorn, Millar, Mueller, Lowell, J.D. Drew, Cabrera and Matsuzaka. (NOTE: That list of Red Sox players includes those who contributed to either their 2004 or their 2007 championships).