Gore still hopes to get Green votes without walking the Green walk. He still believes that he will lose more votes by supporting Green issues than he will gain by not supporting them. And if Nader drops out or if Greens give in and vote Gore, he will have been right, and he will not have any reason to believe that he needs to support Green issues to win in 2004. One of the nice things about our electoral system is that regionally significant third parties can have a major effect on the outcome of elections. They only need to pull enough to swing a few states.
Some states are worth more than others, of course.
Any party whose only success was swinging California away from one candidate, for example, would see its platform gobbled up by the other parties. Sometimes you have to support the foot in the door. But if you believe that Bush's position isn't strong enough, you still want to vote for someone who holds a stronger position. Marijuana prohibition is a good example of what happens when a vote is seen as unswingable. Every state that has taken up the issue of medical marijuana as an initiative has voted, usually overwhelmingly, in favor of letting doctors prescribe it.