I think it's a product of the typical nerd black-and-white thinking. People are either raging fundamentalists who take the Bible as literal truth and reject all science that contradicts it, or they are reasonable people who think the Earth is old and evolution happened.
In reality, there are a ton of creationists who aren't raging fundamentalists. They aren't creationists because they believe so fervently that they cannot accept any contrary evidence. They're simply creationists because that's all they know. Many of these people, when exposed to the truth in a constructive way, will change their minds.
In fact, I was one of those people. My elementary school taught creationism and I believed it because, well, why not? Later on I found out about the scientific explanations, and came to accept them instead.
You'll never convince Ken Ham. Nor will you convince his hardcore supporters. That's not the point. The point is to reach out to people who aren't hardcore about it, but who still believe nonsense simply because they don't know any better. And what better way to convince them to listen at all than to involve a religious figure arguing for the other side? How many of these non-fundamentalist creationists would tune in to "Bill Nye Explains Evolution", versus "Bill Nye Debates Ken Ham"? You'll convince lots of them.