Millions of people either deny that the Holocaust ever happened and a great many more that it happened to the extent claimed. If things are so clear cut about the Holocaust, this would not be a subject of discussion. Some of people point to the wide range of "official" numbers, some are angry because not only Jews were killed, and some are angry that Holocaust survivors and their families have received and continue to receive large cash settlements. To deny a debate on the most fundamental, undeniable fact - that the Holocaust occurred - would simply feed the flames of doubt. Who is getting these settlements? Why are the International Red Cross numbers different from the U.S. military numbers, that are different from the Russian numbers, that are different from the Nuremburg trial numbers, that are different from the Auschwitz memorial numbers? Why are people arrested in Germany for asking questions? Sure, in the U.S. and other places where there is room for open debate, there are no problems, but in Germany in particular I can see why some people would continue to be iffy on the subject. There is, unfortunately, plenty to debate.
Yes, there was a Holocaust, yes, people were killed en masse, but why not answer the nagging questions with evidence so that people can have all of the facts so all of these nut jobs can be made to shut up?
To refuse to debate on the topic and set the record straight would be awfully suspicious in the eyes of those millions that say it never happened. The best way to garner support for a cause is to have someone oppose it and refuse to give it credence. It works for religious cults, it works for unpopular political systems, and it would probably work for Holocaust deniers.
The whole idea is "show me evidence or it didn't happen". If one were to scoff at what to a great number of people would be seen as a legitimate debate would simply indicate to those people that there is something to hide. Besides, who would turn down a debate with the evidence stacked in their favor?