Why seems pretty self-evident.Whether we're talking about Souter or Ginsburg, we're still talking about the same vote on Raich and Kelo.
A president Gore or Kerry would have nominated two left-wing anti-RKBA activist judges. And even worse, Ginsberg and Stevens are old and sick, and would have retired so that they could be replaced by suitable left-wing candidates. Which can still happen if a democrat wins in '08.
Then when the next 2nd amendment case came up, your guns would be gone forever. All nice and legal like.