Many academics are critical of Wikipedia, mainly because students often lift huge chunks from it to put in their essays. Dismissing it entirely, however, is unfair. The editors are striving to improve the quality of articles by adding references and weeding out biased statements and outrageous claims. As the editorial work is done by volunteers, this takes a lot of time. Many articles are written by academics (and are properly sourced) and those that are not properly referenced or are biased have a big warning displayed at the top of the page. People who are critical of Wikipedia should get involved and edit the articles, not moan about it.
I tend to use it as a way of gaining an overview of any particular topic, in much the same way as I use the Introduction to... series of books, or the Beginner's Guide to... books. It is not the done thing to cite those in essays either, but they are very useful.
The root problem with student essays is not that they cite Wikipedia or are guilty of plagiarism; the problem is that they don't know how to write properly, so they turn to these means to get the work done. Do they still teach the arts of précis, rhetoric, logic and grammar in schools? I think not - and that, it seems to me, is the source of the problem.