1. This has typically been "banned" from an educational perspective.
2. What you quote, may not be there later...
They way that I do use Wikipedia for writing is simply as a reference. It's great to get an understanding of a concept quickly. Also, there is a works cited section at the bottom. This is wonderful, because it can give you ideas where to look for the information you need.
When it comes to scholarly material:
Wikipedia is a good launch pad, but not a good place to land.