John Anderson shared on Facebook the wonderful threat he makes to students if they should dare to cite Wikipedia in an assignment.
He said he tells them he will change the Wikipedia article, penalize them for citing Wikipedia, and then penalize them again for not citing it accurately!
The point he is trying to get across, of course, is that Wikipedia can be written and changed by anyone. The combination of its instability and the fact that one has no way of knowing whether at a given moment it was written wholly or primarily by experts in the area in question makes it unwise to use, except as a stepping stone to actual scholarly sources.
I do not have a problem with students using Wikipedia in the manner I just indicated, i.e. to lead them to other sources. The problem is in relying on someone else to mediate actual content and information to you from experts. Now that so many sources which reflect genuine expertise are available online, there is simply no reason to be relying on non-experts to read the experts for you and tell you what they say, if you are trying to really understand a topic.