Sanctioning the devil

Sanctioning the devil is a logical fallacy that occurs when Person A avoids debate with Person B because debating B would (supposedly) give B undue credit or "sanction" their views.

It is essentially a fallacy of opposition and an informal fallacy.

Form
Sanctioning the devil follows the form:

This argument is valid so long as the premises are valid. The fallacy most often occurs in P1 (debate does not lend credibility, if done correctly) but may occur in P2 (treating some view as inherently wrong and unworthy of consideration).

Inverse form
The inverse form of the fallacy (my opponent is too afraid to even debate me!) is very common among creationists and other cranks.

P1 is simply incorrect (in general and in specific — see the exceptions section), and so the conclusion does not follow.

Explanation
Debating an adversary, or "sanctioning the devil", supposedly shows that their position is at least partially credible, while denial of the opponent indicates that their arguments are not even worthy of consideration.

However, while avoiding the debate and presenting your viewpoint as a unified message is an effective strategy, it is not (according to empirical studies of people observing) as strong as presenting your viewpoint and refuting one or more alternative views.

Avoiding debate altogether lets the undebated opponent paint their view as ironclad and irrefutable &mdash; or why else would their cowardly debate opponent run from the debate?

Exceptions
It is acceptable to avoid debate if one knows that one is not prepared. For example, many evolution proponents are not up-to-date on each kind of creationist argument, and may feel unable to accurately represent science.

Alternately, one may avoid debate if one knows it cannot produce useful knowledge. For example, many creationists simply Gish Gallop their opponent out of existence; this mistakenly gives the impression of creationist success.

Debating an opponent repeatedly when the opponent has clearly failed to prove an argument and has no new evidence devolves into the balance fallacy.

Examples

 * We are not going to give this [insert loaded language/slur here] a platform!
 * Very often, proponents of evolution refuse to even debate creationists, on the principle that "That would look great on your CV, not so good on mine".
 * Even more often, creationists claim that evolutionists don't debate them because evolution is false.