Why are some professors critical of even casual use of Wikipedia?
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).