Opinion pieces are not (or at any rate, should not be) cited as evidence, except as evidence of the opinion of the people who write them. They may point in the direction of evidence, and in some cases the fact certain people (eg Colin Powell) have particular opinions is significant in itelf..

That applies whether the opinion concerned is that of a someone posting on the Mudcat or the Daily Kos or Move ON, or an editorial writer in a newspaper.

The essential principle should always be that of CP Scott, who wrote in 1921, on the occasion of his 50th year as editor of what was then the Manchester Guardian, "Comment is free, but facts are sacred."