Reliable or not so reliable - Why is the majority of teachers or professors dubious about their students using Wikipedia?

From Simple English Wikipedia: Wikipedia aims to be the ultimate learning resource, the sum of all human knowledge. Yet many students may have difficulty understanding the complex language used by the English Wikipedia. They may require a simpler version, which is easier to understand and which should still provide them with the information they need. The Simple English Wikipedia is an encyclopedia to help make this happen, to provide students and learners of English with a simple, free resource.

Concepts such as open source, copyleft, collaborative writing, and volunteer contributions for the public good are sometimes new and unfamiliar ideas in educational settings. Wikipedia offers an opportunity for educators to explore concepts of public trust which are likely to continue growing in prominence throughout the lives of today’s record population of youth.

Is Wikipedia accurate and reliable?

Generally, yes, Wikipedia is accurate. Wikipedia is developing rapidly, the reliability of the encyclopedia is improving all the time. Because readers compare articles to what they already know, articles tend to become more accurate and detailed. Certain articles about many of the major sciences were developed from other free or public domain encyclopedias. This provides a reliable basis upon which encyclopedia writers can develop more current information. Wikipedia is cited almost daily in the press.

On the other hand, it is possible for an article on Wikipedia to be biased, outdated, or factually incorrect. This is true for any resource. One should always double-check the accuracy of important facts, regardless of the source. In general, popular articles are more accurate because they are read more often.For this reason, factual errors are spotted and corrected in a more timely fashion. In particular, there may be a Western bias since that is where most contributors are from.

http://simple.wikipedia.org/wiki/Wikipedia:Schools/Teachers%27_Guide

What is your opinion? Is Wikipedia overrated? Is there a danger that libraries will soon be seen as uncool or outdated?