A small sidenote to start things off:

If I tell them "I read from Wikipedia that..." I get dismissed immediately, yet in online forums we use it like a Bible.

Well, one of the reasons for that is that "I read in Wikipedia" is almost synonymous with "I have exactly 5 minutes worth of knowledge on the topic". The problem here really isn't the fact that you read Wikipedia, but that citing from it implies that you have read nothing else on the topic. If I am an expert in whatever field, I would probably not take a concern from somebody who implies that all his knowledge comes from a few-minute Internet recherché very seriously, either (no matter what source (s)he actually found). Also, which online forums "use it like a Bible"? Most that I hang around at are very critical of Wikipedia quotes, mostly for the reason I stated above - arguing based on a Wikipedia entry does not exactly establish creds as a person knowledgeable about the subject.

Now, let's discuss the real question here:

What is the real reason Wikipedia is perceived negatively among many professors, even for informal use (e.g. as an introduction to a subject)?

(note that the question is specifically about using Wikipedia as an introduction to a subject, not as a primary, citable source)

Honestly? It is probably a combination of feeling threatened, reluctance to embrace change, and lack of knowledge how Wikipedia articles actually evolve over time.

"Feeling threatened" in the sense that Wikipedia is kind of decentralising knowledge compilation, which is of course not necessarily something that makes academics (the people that used to be more or less the definition of "compiled knowledge" in pre-internet times) very comfortable.

"Reluctance to embrace change" in the sense that Wikipedia is (in comparison to text books or lectures) a very new (and radically different) way to get an introduction to a topic, and most humans tend to be sceptical of this kind of disruptive technology.

"Lack of knowledge" in the sense that many critical academics simply have not taken the time to study how (especially popular) Wikipedia articles actually evolve over time. I am convinced many would be positively surprised if they knew how well quality control in Wikipedia actually works in practice. I remember that in 2004, c't (a well-known German magazine widely read by IT professionals) ran an experiment where they took random articles out of various encyclopaedias, anonymised them so that one could not tell the source anymore, and had domain experts compare them to anonymised Wikipedia articles for quality and technical errors. Wikipedia was consistently rated higher-quality than even well-respected standard encyclopaedias. That being said, I assume that the average quality of Wikipedia articles degrades a lot for entries on more esoteric topics, so I actually agree that for deeply scientific topics, one should be somewhat skeptical of Wikipedia, just as one would be about any other single source.

Finally, I have to say that I know many professors that don't have a problem with using Wikipedia as a starting point for your review of a subject. However, if you write, for instance, an seminar paper, you are expected to read the primary sources (and I fully agree with this).

I think the primary reason professors don't want students to use Wikipedia is because a lot of students only quote Wikipedia instead of actually researching a topic.

The great thing about Wikipedia is it can give you a general idea about a topic and offer a starting point to dig in deeper. However, students can be lazy and instead of digging in themselves, they take the easy route and just reference Wikipedia. This is obviously not the point of Wikipedia and research, and I can understand the need to announce on day one to not reference Wikipedia.

However, the reference and bibliography sections of Wikipedia is the real gold mine and could be a great starting point for any research topic. This is what I see as a pure advantage of Wikipedia and what professors should also say on day one.

For example, suppose I'm interested in Financial Economies, so I do a quick google search. First hit : http://en.wikipedia.org/wiki/Financial_economics

Great discussion of the discipline, vague descriptions of risk, graphs, and financial pricing theory. But the real gold mine is in the reference and bibliographic section. There are references for financial economics, asset pricing, and corporate finance, which a great starting point for looking further into topics. The links point me further into a direction I'm interested in from very famous authors.

I can understand the question as a student and I think it's the professors responsibility to explain how not to use Wikipedia and how to use Wikipedia.

The very fact that you need to ask this question, in a way, provides its own answer.

One of the primary functions of academia is to teach the skills of research. There are two aspects to this, both critical; first, being able to find what work has been done by others and, second, to do new work yourself. Without the ability to effectively do the former you risk wasting time repeating pre-existing work when it comes to the latter.

Now, for non-academics, Wikipedia has rapidly become a catch-all, sole source, and generally fantastic one-stop shop for information. While this is well and good for casual use, it nevertheless provides an enormous disincentive to acquiring and practicing those critical skills of research that you will need if you intend to continue to pursue a career in academia.

My feeling is that most professors are, even if subconsciously, objecting to Wikipedia for this reason above all others. It feels wrong to them because it is a shortcut -- a cheat that puts a stop to a student's research effort before it even begins. In their own careers they have a deep appreciation for the need of strong research skills and, likewise, an appreciation for the need to teach those skills to students.

If Wikipedia ceased to exist at this very moment, ask yourself the question - "Where would I find information, and how would I go about doing it?"

In the world of academic research, this is the situation you find yourself in - beyond a certain level, Wikipedia will not have the answers you are looking for and you will need to have developed more advanced research skills to find them.

By artificially outlawing Wikipedia, professors are attempting to simulate what the real academic world is like - one where the answers are not already known and easily accessed; one where you, the researcher, are tasked with needing to know how to effectively dig deeper to answer questions for yourself.

The critical thing to realize is that higher education is not like primary school anymore. The error you are making is in thinking that an assignment about topic-X is chiefly intended to populate your brain with information about topic-X and that the most effective means of getting information about topic-X into your brain is the best solution to the problem.

This is wrong.

Topic-X is largely irrelevant. The real task is to teach you the skills you require to find information about any topic. Topic-X is simply a convenient and concrete sample of a topic on which to learn and practice those skills. That the information on Topic-X is readily available on Wikipedia is merely a reflection of the fact that, as a junior academic and undergrad, you simply (at the moment) lack the technical education necessary to be given a more advanced "practice" topic to research - one that would not be so readily found on Wikipedia.

Nevertheless, the professor's objection to Wikipedia is for a very clear reason - it is entirely counterproductive to their primary (and probably unstated) objective of getting you to exercise and develop real research skills.

Consider the broader context.

From a slightly different perspective, in a lot of ways Wikipedia has really raised the bar. If you are a university student and you are working towards a degree in a subject then it bears considering what that means. If any joe public can look something up on Wikipedia with the most minimal amount of effort then what does that mean for you?

Surely an academic degree needs to be something much more than a certificate proving that you know how to type "X" into a wikipedia search box. A child of 6 can do that these days - if you're looking to gain a serious academic qualification then you really need to be going above and beyond what has become this most basic level of ability to research information.