There are some people who are such raving demagogues that even the concept of debate with them, particularly an informal debate, is ludicrous. Against a verbal barrage, reasoned arguments get lost in the noise, and take longer than the sound bites that demagogues typically deal in. For example, as a conservative I would probably avoid "debating" someone like James Carville. I'm sure liberals can think of a few people they wouldn't butt heads with either.

I don't know Le Pen well enough to know if he falls into the Carville motormouth category, but if he does, I probably wouldn't debate him either.