Before I was barred (without any explanation) from posting, I used to spend a lot of time over at Crosswalk Forums, often debating evolution and the age of the earth. At first I thought that there was so much evidence in favor of the accepted scientific view that I would certainly be able to convince the doubters. But I soon realized that there were many ways people evaded the inevitable. Some just buried their heads in the sand and denied the evidence, or said it was unreliable, or faked, or whatever. But others were actually thoughtful, and even well-informed. One in particular was studying biochemistry, and knew much more about genetics than I do. Yet he refused to give up his anti-evolution stance.
Still, it was an interesting intellectual exercise to try to present the evidence in the clearest possible way. And, I thought, even if I wasn't convincing any of my debate opponents, maybe some of those lurkers were getting something of a clearer view of science and its methods.
Just around the time I got banned, someone else posted a poll asking how many forum participants had gone from young-earth creationism (YEC) to evolution/old-earth, and vice versa, as well as who had not changed their stance. Of course, there were many YECs who had remained YEC, as well as those who had been and remained convinced of evolution.
What really surprised me, though, was that there were more who had become convinced of evolution through participation in the forums, than had gone the other way. This in spite of the fact that every argument I posted was immediately inundated with links to, or arguments from, Answers in Genesis and the like.
So I feel strongly that, no matter how useless the debate seems, no matter how unyielding our debate opponents may be, this kind of debate makes an impact.