Things changed. Several reviews on the reliability of Wikipedia concluded it is about as accurate as more standard encyclopaedias, while having more content. I too have heard that Wikipedia should not be used at all when I was in school, but nowadays, at least at the universities I have some acquaintances in, professors do not hesitate to point to Wikipedia when they consider an article is pertinent and of quality. Thus, fears about the collaborative nature of Wikipedia being a major source of unreliability now are seemingly fading away: on the contrary, it leads to more content, with an approximately equal degree of accuracy as others encyclopaedias.
That said, Wikipedia, as well as other encyclopaedias, should never be used as references. This is because of the intrinsic nature of an encyclopaedia, which produces inherent limitations.
An encyclopaedia is meant as a compilation of the current knowledge. How is it done? By using reliable sources of information. 620
As such, an encyclopaedia never produces new information, it only summarizes what was already done by others. In addition, to increase reliability and ensure notability of inclusion, only secondary sources (eg, journalists articles) can be used, rarely primary sources (the original source: a book, an interview, a talk, etc). An encyclopaedia is itself a tertiary source (so you can see we are getting further and further from the original information...).
This leads us to solve a common misconception, and the probable reason why encyclopaedia are frowned upon as sources for any serious work: they can only ensure verifiability, not the truth.
Indeed, since encyclopaedias can only compile knowledge, never create it, they are often limited to the current common knowledge, and not the state of the art. In addition, the usage of secondary sources might (heavily) bias the content included: a notable falsity cited by lots of reliable journals will be included whereas a truth only published in a book will not be notable enough to be included. Both of these inherent flaws are most apparent in fast moving information domains, such as articles on political events or figures, as well as contemporary history.
To take a somewhat extreme illustration, a poll on the french Wikipedia asked "whether, if Wikipedia existed a few centuries ago, it would state that the Sun revolved around the Earth". The majority (84%) answered yes.
In the end, Wikipedia, as well as any other encyclopaedia, can be a great first low-ceiling entry to a new knowledge you want to get, but it can only be seen as a gate to dive deeper into the included references, as well as the non-included references that you can find by your own means, which is particularly important if you are looking for the state-of-the-art (instead of only the current common knowledge). 111