You would not debate a topic that you were not an expert about? That's foolish?
Here is a practical answer for why that is NOT foolish.
JW's claim to know the truth, should a person have to study and learn all there is to know about JW's before they can debate and challenge their beliefs?
Does it take understanding the entire doctrine of the faithful and discreet slave for a non JW to just ask questions and probe and poke holes into the logic of it without them ever becoming indoctrinated?
Infact if anything you should be encouraging such debates. All people have priorities and a limited amount of time. With work, life, the billions of books on anything imaginable to research, thousands of different religions and beliefs, etc. Perhaps with such debates, some people never had any intention of reading books on the subject because they dismissed it or it was low priority, but either lurking or those involved in such debates force them to do more research and maybe grab a book and bump it up the list in their priorities.