I was going to type this as a response to an earlier comment but it ended up being too long. Here goes:
In a normal season, Giambi is clearly a better hitter than Posada. More power, better on base percentage, and generally better average. And if anything, he might be a step faster. For some reason Jorge is having a career year at a very old age (for a catcher), but let's ignore that for a moment.
So by your logic, do you think the team would be better off with someone like Jason Kendall behind the plate who can run and bunt and go first to third, or would you rather have Posada who gets on base 10% more often, hits homers and doubles, and drives runners in -- but grounds into a lot of double plays and can't always score from 2nd on a single?
The name of the game is to NOT make an out since you get so few opportunities, and Giambi does that better than anyone. And if he's playing DH -- who is he really taking at bats away from? Damon? Duncan? He's clearly a better hitter than them, and I think he's makes the lineup stronger without a doubt. The caveat being "if he's healthy." He's definitely the kind of guy who can't hit a lick unless his body feels right -- but that's up to Torre/Cashman to decide.
As for the Yanks winning without Giambi and during ARod's slump, I believe its naive to think there is any correlation. They've been on a tear because they've been playing crappy teams that can't pitch, and our own pitching has been better than it was early in the season. When they start playing Detroit, Cleveland, Boston, and Anaheim again is when you need guys like ARod and Giambi playing well for the offense to put some runs on the board. Shelley Duncan is not going to hit 5 homers in 22 at bats against Beckett, Dice-K, Gagne, and Paps.
So in summary, I guess we'll agree to disagree on the value of both ARod and Giambi to the team's overall success.
But you know what we can all agree on? That Kyle Farnsworth should not pitch in another game. That guy is TERRIBLE -- and he's turning into a clubhouse cancer. They need to get rid of him, even if it costs them $$$$.