Recently, I made an answer to one of the questions of this site. When searching the sources with the aim of finding the answer, I came across a complete and referenced answer from Wikipedia. As you know:
1- Wikipedia is a free encyclopedia under the Creative Commons Attribution-ShareAlike License which allows sharing and/or remixing under certain conditions.
2- Texts in Wikipedia has no certain authors. The text might be edited by almost every one.
3- Verifiability is one of the three core content policies of this encyclopedia and probably most of texts are referenced using citations in text.
Note the answer contains that no plagiarism or copy-right violation, based on the above points. Surprisingly, a mod deleted my answer and commented as such:
He has mentioned two points in his comment:
Firstly the answers should not contain plagiarism or copy-right violation and as you know the text is free for sharing!
Secondly, the quotes must be clearly attributed to their respective authors. As I said, texts in wikipedia has no certain author. Moreover, I mentioned at the end of my answer that the answer had been quoted from the Wikipedia!
There is no plagiarism, there's no copy-right violation! So, Why shouldn't we use materials mentioned in Wikipedia?