I agree. Actually, questions are useful at any time. When it seems like the person is changing their position without acknowledging it, I usually ask something like this:

"I'm a little confused. Before, you seemed to be saying X. Now, you seem to be saying Y. Which is it?"

If the person evades my question, that's the end of the debate as far as I am concerned. I have my own rules of debate, and one rule is that my opponent must answer reasonable questions so that I may understand his or her position.