I routinely reject tag wikis that are copy-pasted from Wikipedia, because they are almost never appropriate. Consider a recent suggested excerpt from my review history on Science Fiction & Fantasy:
Brandon Sanderson (born December 19, 1975) is an American fantasy author. A Nebraska native, he currently resides in American Fork, Utah. He earned his Master's degree in Creative Writing in 2005 from Brigham Young University. He was chosen in 2007 to complete Robert Jordan's epic fantasy series, The Wheel of Time, and his own works have appeared on the New York Times bestseller list.
In addition to the illegality of including Wikipedia content in the excerpt (because of the lack of attribution), most of the information here is completely irrelevant. What the excerpt should say is who that person is (an author) and why he's famous (what subgenres he wrote in, any particularly famous books by him).
The suggestion for the tag wiki goes on for a couple more paragraphs:
Sanderson married on July 7, 2006 and is a member of the The Church of Jesus Christ of Latter-day Saints. (…)
Seriously, how is this relevant? We don't care about his personal life, we care about his writings!
Creating such tag wikis not only doesn't help the site, it makes it harder to eyeball tags that lack a proper wiki. A tag wiki for a tag that isn't ambiguous (it's obvious that the
[brian-sanderson] tag is about Brian Sanderson's works) only helps if it contains useful information, which Wikipedia copy-pastes rarely do. Wikipedia copy-pastes do more harm than good. I'd favor an automatic filter to reject them outright.
Inappropriate tag wikis are a big problem on SF&F. I'm having trouble even finding a good tag wiki to cite as a counterexample (unless I allow myself to cite one I wrote). Here's an ok one, at least it cites some of the most important information without too much fluff.
Tag wikis are mainly intended for people who are about to ask a question; they should help those people write better questions, or find answers on their own. An example of something that's useful in a tag wiki for an author is an overview of that author's works; another is a link to a wiki site about that author.