Well and attaching rings to players in baseball for an argument is pretty dumb too... Yes.. Jeter had some huge moments in big games but one player cannot get a team to a title..
Bonds completely dominated the 2002 World Series for example.. more than any player could ever dominate and the Angels still won. Nomar don't need rings to prove.
I am not saying that Garciaparra leads the Yankees to a title. I agree, 1 player does not have that sort of impact.
But I do believe that he represents a significant improvement over Jeter on teams that won a title (and competed for them).
Considering how close the Yankees were to championships in 1997 and the early 2000s (at least through 2003) it isn't a stretch to argue that Nomar puts them a few notches closer.
With regards to Bonds, I like to use him as an example of sample size.
Up until that 2002 season, Bonds was pretty awful in the postseason. But over time players tend to perform according to their career averages. It was only a matter of time before Bonds would light things up in the playoffs. And 2002 was that turning point.
Even so, Bonds only had 48 postseason games (151 at bats). That's still not enough to make a judgment about his abilities. If he had 158 postseason games under his belt (like Jeter did), I am confident that Bonds' postseason numbers would look A LOT like his regular season numbers.