Some SE sites - Astronomy, Space Exploration, Physics, etc. will allow Wikipedia to be used a source in many cases, although multiple sources are always preferable, especially for really key points. Other sites have different standards. Biology (which harps on references) unofficially frowns on Wikipedia, and on Skeptics, it's almost and obscenity. What should be our stance on this? Obviously, it's better to have multiple sources on an answer. But should we accept Wikipedia at all?
This is sort of a rehash of my answer here in a different context.
There are two main goals of including a link on this site. One is to establish credibility of a claim. The other is to help the reader find more information. In practice, these are pretty easy to distinguish. If you find yourself needing to directly quote or paraphrase from the source that you're citing (e.g. "On this site it says that Gauss found the sum of a finite arithmetic progression in primary school.") then you're relying on the credibility of that site. If that site was incorrect, your answer would be wrong. So you should be pretty sure that the link therein is credible.
On the other hand, if the source could easily be removed from the post without really significantly affecting the main content, then you're providing the link just for the sake of helping users who may not be familiar with certain background material or who may be used to different terminology. Even if that source turned out to be wrong, you could just replace it with a similar source elsewhere with no harm done. For instance, writing "Gauss found the sum of a finite arithmetic progression in primary school" is not in any way basing your claim on the contents of that page; the link here is fairly tangential. You don't need to believe that anything in that link is true to believe the answer. The standards for this kind of link can be lower. The source merely needs to be helpful in this case. However, you'll note that now the claim of the story of Gauss is completely unsourced. If it's something that's of debated validity or not particularly well known (in this case it is fairly well known but of debated accuracy), you still need to provide a credible source. It's also worth pointing out that the fact that the link is tangential is still not a good reason to link to content if it's clearly low-quality or false, as such would not be helpful.
Thus, it's important to make a distinction here between merely linking to or mentioning something and actually citing it as a credible source. Wikipedia is not a credible source, but it may still be a helpful one. That is to say, it can make your answer better if it's already well-cited, but it can never substitute for a valid citation, and a statement only supported by Wikipedia is no better than a completely unsourced one. Your answer should be correct even if the Wikipedia page were completely incorrect. If you need to cite a credible source, it should not be Wikipedia (or indeed any similar freely editable source).
Note that this is using Wikipedia content in much the same way that it's used on Wikipedia itself; when editing Wikipedia pages you can link to other Wikipedia pages for background, but at the end of the day you need credible external sources for any claims made on the page. A good Wikipedia article will cite its sources, and those should be more credible (if less complete) than the article itself; those can be used to cite specific claims. If a claim on Wikipedia is not cited, not only should you not cite Wikipedia, but you shouldn't even believe Wikipedia until you can find a credible source (not just copying from Wikipedia!) saying the same thing.
With that said, not every fact needs a citation. Generally, citations are required when the point is debated or controversial, mostly unknown, and for direct quotes. You can safely say "Gauss was born in 1777" without any citation; this is a well-known fact that can be verified in many places. Some, but not all, statements on Wikipedia are of this type.
To the best of my knowledge, this is basically the policy everywhere on SE except Skeptics, where the whole point of the answer is to find a credible source and thus there's essentially no room for an uncredible source like Wikipedia in the first place. It may be that some sites like Biology have problems with a lot of people incorrectly citing Wikipedia as a credible source, and so they police links more strictly, while sites like Physics don't have this issue either because they have fewer claims requiring citation or because people on these sites more commonly cite credible sources like academic journals. Given the (IMO) highly variable quality of Wikipedia articles on History of Science and Mathematics, we should be similarly discerning with such references, but I see no problem with people tangentially linking to background material on Wikipedia any more than any other reference source so long as it isn't used to establish credibility.
My answer here is an example of an answer that uses Wikipedia as a source but relies on others as well. I checked the relevant information with two (three, though I didn't directly explain the third; perhaps I should) other sources and explicitly stated that I did not want to use Wikipedia as a primary source.
I'd be okay with answers like this. My primary purpose in using Wikipedia was simply to have another source. On its own, it would not be too credible. But it is consistent with what other sources say (I wasn't able to find anything against it), and appears, after quite a big of digging around, to be accurate.