Zito is a good pitcher, not a great pitcher. And it's true, if he's your three strter then you've got a good staff. However, if you drastically overestimate what your #1 pitcher is capable of then it LOOKS better than it actually is. If the sox had a rotation of Pauley, Beckett and Schilling I could say Schilling as a #3 is tremendous. But RJ isn't doing anything and barely deserves to be called a #1.
Zito would be a #2 for the Yankees, in practice this year, in theory and in practice next year as I'm guessing RJ will retire after this season.