Well, Gardner brings excellent defense and extreme speed on the bases. There is a lack of power, but also other benefits to having him in the lineup.
Assuming Jeter isn't able to continue to hit like he did this year (and if he does, forget it, play him at DH), he'd be a bad choice at DH because there will be dozens of other players who can do the job better. Guys like Carlos Quentin, Josh Willingham, Billy Butler, David Murphy, even Adrian Gonzalez could probably be had by the Yankees over the next couple years and they are all likely better hitters in three years than 41 year old Jeter.