Haha! I stopped thanking people after reading a previous post like this - from you. And I only started it (briefly) because I'd noticed others doing it and thought perhaps I was rude not to.

But you make an excellent point. I only thank people if they've actually brought an interesting new angle to the debate and I'm grateful for the effort they've made to do that.

Otherwise, I just respond appropriately like you would in a normal conversation. When you don't normally thank someone for speaking to you.