Wikipedia is not a source; it is a compilation of sources. Other encyclopedias can be sources because they are backed by the integrity of expert review and editing. Usually they feature articles written by experts in not only their field but in their subject matter, all double-checked and edited by leading academics. Wikipedia has the occasional expert, but it is constructed in a fundamentally different way, much more like a database than an encyclopedia. The main problem is that, while given information may be sound, it is seldom the whole story. Many article talk pages have a section or two saying "Hey, I found these cool sources, how can we work them in?" This sort of information is circumstantial, rather than reliable. The sources Wikipedia uses, however, may be acceptable in academic settings (except for all the news articles).