Posada had a weak arm, but was an above-average pitch framer and excellent game caller. Bernie Williams was a decent CF, and Jeter was an average SS who made all of the routine plays and had a howitzer attached to his right shoulder. Not a shining example of terrible up-the-middle defense in WS winners.
Also, using one (albeit mostly incorrect) example doesn't prove that statement. Up-the-middle defense is extremely important. Look at the way moribund pitchers play "up" when they reach the Cardinals organization. Molina gets a lot of the credit, and with good reason. A good caller and framer can prove the difference between winning and losing in tight contests.
On the other hand, you can hide a weak SS or CF by surrounding them with good fielders, but how do you hide a weak catcher?