I wish professors would stop saying to students "Do not use Wikipedia as a reference."
What the professor means is "Do not read Wikipedia. Do not take ideas from Wikipedia. Instead, consult scholarly journals, books, government documents, and other serious, credible, high quality sources."
What students hear is "Don't put Wikipedia down in the list of references at the back of the paper."
So it comes as a shock to students when they end up in the Associate Dean's office, confronted with an alleged academic integrity offense, just because they've failed to cite Wikipedia properly.
Whatever a professor says, students are going to use Wikipedia. They use Wikipedia for the same reason that professors and journalists and bloggers and people who are curious about life use Wikipedia: it's free, it's at the top of the search results, and it often provides a short, reasonably accurate general introduction to a topic. No, it's not completely correct, so it's a good idea to double-check anything found on Wikipedia against other sources. But it's a reasonable place to start.
What professors need to say to students is: "Every source you draw ideas from - Wikipedia, the lecture notes, the textbook - should be acknowledged in your footnotes and/or your bibliography."
And students need to experiment with a new and radically different approach to essay writing: disconnect the internet, go for a walk, think of an idea, and write it down.
(For more on this topic, see Nick Rowe's post on Student Essays as Collages.)