As a university instructor, I disagree. Many subjects are presentable in sensible sequence with knowledge neatly building on prior knowledge. The entire curriculum is created with prerequisite and co-requisite courses. Attempting a 400 level class without having mastered the 100 level course content is a recipe for pointless struggle.
Wikipedia is a great resource. It's articles are self contained and generally rely only on general knowledge. Wikipedia is not a good source for delving deep into subjects.
Having said that, I use wikipedia extensively. There is almost an entire Computer Science undergraduate curriculum in there. t still requires a guide. There are still sensible paths through the information. Here is a great example: http://en.wikipedia.org/wiki/Fibonacci_heap In the context of a Data Structures course, that is a great page. What benefit would that page provide to somebody who has never programmed and doesn't know what a data structure is? You can follow links to http://en.wikipedia.org/wiki/Heap_(data_structure) and then http://en.wikipedia.org/wiki/Data_structure and then you are directed to dead-tree textbooks that explain the topics in a sensible sequence.