Evolution supporters decline to debate cdesign proponentsists.

     Actually, there are a number of good reasons to decline such a "debate." Creationists do not present evidence. They hop around between different ideas (like cosmology and abiogenesis) until their opponent admits lack of expertise in a specific field and then say "aha! godidit." It can detract from productive research. However, one bad reason was advanced by an evolution supporter. (I forget who, exactly. But I am speaking on general principles.) This particular person did not wish to "lend credibility" to creationism by allowing debates.
     That's right. It was an admission of an effort to regulate public perception of an idea. The tactic is time-honored and will probably backfire in time. Power groups use that type of tactic so that the masses remain unaware of reasoned dissent. I don't know why it was used there as creationists do not use reason. Efforts to suppress dissent generally cause the public to wonder what the suppressors are hiding. If your intellectual adversaries are crackpots, give them every opportunity to speak their piece. They will reveal themselves.
     Ultimately, attempting to avoid "lending credibility" to creationists will only help the creationist cause. Similarly, I expect that within 100 years or so, few people will believe the Holocaust happened. I do not predict that people will consider it a good thing that they would want to emulate. But people are likely to dismiss it as official propaganda. The reason is simple. In Europe, it is illegal to assert that one does not think the Holocaust happened and/or give reasons why one believes it to be fiction. Eventually, direct confirmation will be impossible. Already much of the evidence could be faked. When the only thing left that can be confirmed is a propaganda tactic, people will not take the event seriously.
     Suppression of dissenting views is a tactic suited only for those who are enemies of truth. Truth needs no such protection. Full inquiry is best path to protect truth.