Why is he doing this?
Because Nye is, at heart, an educator. He actually does want to help people understand the science. I'm sure he knows that the deck is stacked here, but he also knows that this debate will be posted on youtube and that there will be people who see it that honestly just don't understand the issues. That's his target audience.
Will it lend a bit of credibility and publicity to Ham? Sure. But if you want to actually change the public perception on this issue you gotta go where the public is. Maybe Nye will do better than expected - after all, he made his career on explaining complicated scientific ideas in simple ways that people could understand.