Here's what I found in Dennis Tucker's 1989 Finding Religion (in the Library):
"'For your research paper, you must use three different sources and you can't use an encyclopedia.' Surely every student has heard this from his teacher many times over during elementary and high school. Why the bias against encyclopedias? Are they inherently evil? What's wrong with them? Actually, nothing is basically wrong with using encyclopedias--good ones anyway--if they are used properly. But fifth graders (and sometimes college students and seminarians) tend to rely on them too completely.... Students use them heavily because they want their research pre-digested for them rather than doing it themselves from primary sources." (32)
Sounds exactly like the reasons to not use Wikipedia. Tucker goes on to talk about encyclopedias as summaries and bibliographic tools--a starting point, not really a source. Again, an apt description of Wikipedia.
So perhaps educators should take a break from the refrain of "Don't Use Wikipedia," and instead encourage its use for its actual purpose--a starting point, created by collaboration.