It's always interesting to hear a public debate because it has the virtue of exposing people to a way of thinking they're not normally willing to listen to. Perhaps if nothing else, there's value in that. As a platform for getting to truth, however, it makes as much sense to put credence into a debate performance as it does a news soundbyte.
What I find much more enlightening, however, is a more conversational approach to debating. Instead of having two people giving set presentation, have them talk through issues and try to come to a point of understanding. One great example of this was a dialogue between Richard Dawkins and Richard Harries, where I think there was much more to be gained out of it precisely because of the format. It's not about one-upping the opponent, nor about overwhelming the opponent with numerous objections, but a way to highlight contentious issues and work them through.