Archival articles
I think it depends on how the outside source is being used. If the point of the wiki article is to archive the outside source, because the outside source is inherently itself valuable, then editing should only be performed by the original author (and even then should be discouraged). For instance, you don't try and correct spelling or grammatical errors on the US Constitution because the thing itself is a historical document and has intrinsic value in and of itself. However, it should be made clear that the outside source is outside the purview of ordinary wiki review and may contain opinions that are not widely agreed upon or controversial.
This gives the original author the freedom to have his or her opinions expressed in the original form, without "clarification" or "correction" by other users that may or may not represent what the author originally intended. However, it also gives the wiki to distance itself from certain statements that might not go hand-in-hand with other wiki policy, like if the original source contains an editorial on "Why TVWP should be considered semi-canon..."
General wiki articles
If the point of the article is factual accuracy rather than archival reasons, then it should be opened up by modification by other users besides the original author for the original source. Putting a new article on the wiki should be understood to be an implied consent to the modification of the article by any other users, unless the aforementioned disclaimer is added so the article falls into the first category.
I don't think special treatment should be given to people just because they wrote an article in the wiki, unless the article was put in the wiki without their consent. Otherwise it opens the door to people stating whatever they want ("Inferno is canon!"), and then claiming that it can't be changed because they wrote the article.
