Wikis and Colledge-/level Composition

The article that we read written by Warschauer and Grimes about Web 2.0 applications is very informative for it briefly describes the main characteristics of the most common Web 2.0 applications. Particularly what I found interesting is the comparison of Wikipedia and the Encyclopedia Britannica. And I have a simple question about it. Since the former is not worse (well, there is a very small difference) than the latter, then why don't college professors allow their students cite the Wikipedia? No one would think of dismissing Britannica as a source of citation, and it seems that a team of professionals 'keeps an eye' on the texts posted there. And when I say 'students', I mean students in lower-level classes (the first and second year students).

Involvement and Motivation

Some of the points in Other-Repetition as a Resource for Participation in the Activity of Playing a Video Game are very clear, some are arguable. But one thing that is clear is that all the case studies the researchers look at in this article involve individuals up until 14 years of age. The question that I’ve always had is how motivated are the learners of a FL? To clarify, I’m talking about intermediate to advanced learners who are at least in their mid-twenties (with some of them being in late twenties and early 30-ties).
I understand that gaming is an additional way of learning extra vocabulary but still… I wonder if the results yielded in researches involving teens and individuals in late 20s – early 30s are similar.