There are a couple of schools of thought on this.
One is that guys like Nye shouldn't debate guys like Ham, because simply by dignifying people like that with a response, you're acting as if the two schools of thought are on equal footing.
But I think I get where Nye was coming from.
Because a shitload of people watched this thing, and are still watching this thing.
And Ham's not going to convince a single person that the Earth is 6000 years old who didn't already believe it.
But Nye might manage to cause a few people who DO believe it to doubt it for a minute.
I'm not saying it'll be very many. But if Ham's score is zero and Nye's score is nonzero, then Nye got what he wanted.
(Which isn't to say Ham didn't get what he wanted. He got publicity, and that means more money.)
I think Bill Nye was quite deliberate in his choice to agree to a ridiculous debate with a ridiculous person on his ridiculous home turf. I think he's doing what he's always done -- trying to explain science in very simple terms, to children.
Because I think they're, as always, the people he's trying to reach.
You gotta figure the Young Earth Creationists tuning into this have pretty ossified beliefs and aren't likely to change them. But if they have their kids sit down and watch it with them? Their kids might come away with something a little bit different.
Nye did a pretty good job of hammering three major flaws in Young Earth Creationism:
1. It relies on the premise that God changed the laws of physics following the Great Flood.*
2. It is not predictive. Actual science doesn't just try to explain what happened in the past, it accurately predicts what's going to happen in the future.
3. Even assuming you accept #1, the variation among humans and the variation among animal species do not support the premise that all modern humans are descended from 4 couples and all animals are descended from 7000 types. Even Ham admits this outright and doesn't even bother with more than a half-assed "we don't know how God did it" explanation. Which is really something considering how much he's bent over backwards to come up with convoluted explanations for everything from dinosaurs to the speed of light.
Ultimately, I quite like how Mark Evanier summed it up:
I've read that in the last section, when the two men were responding to questions from the audience, one was "What would change your mind?" Mr. Ham answered, "Nothing." Mr. Nye said, "Evidence." That's pretty much the whole discussion right there.
* I do admit to loving this as a narrative device. Tolkien uses it in The Silmarillion -- after Sauron convinces the Men of Numenor to challenge the Valar, the Valar make the Earth round so that Men can no longer reach the Undying Lands by sailing west.