MLA3 says: "In the past, [we] recommended incuding URLs .... Includsion of URLs has proved to have limited value, however, for they often change, can be specific to a subscriber or a session of use, and can be so long and complex that typing them into a browser is cumbersome and prone to transcription errors. Readers are now more likely to find resources on the Web by searching for titles and authors' names than by typing URLs. You should include a URL as supplementary information only when the reader probably cannot locate the source without it or when your publisher requires it." (6.7.1., p. 212)