Varitek is irreplaceable. Plain and simple. No one else has the same combination of defensive ability, contact, and a bit of pop as well. Sure Piazza might be able to play catcher once again, but he sucks back there. Just a complete liability. I'd rather have Varitek any day.
And on the Magglio issue: he basically broke himself this year. People thought his injury was career-threataning, and many still think he'll never be the same again. I would rather have Manny's offensive presence, defense, contract, and all, than $10 million invested in someone who may never even play again.