The entire point of a debate is to be hit politicians with questions they weren't expecting. They tend to devolve into meaningless, tedious, shouting matches, but the idea is to force a pair of potential elected officials to think on their feet and address issues of public concern. Specifically, the goal is to batter them with questions they didn't anticipate, and see if they can generate coherent, articulate, responses.
The goal is not to hand them the questions in advance so that their writers can formulate a pat, focus-tested, response.