The MLA seems to have stirred the pot with it’s 7th edition of the MLA Handbook for Writers of Research Papers.
In the past, this handbook recommended including URLs of Web sources in works-cited-list entries. Inclusion 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 instructor requires it.
I agree with Maurice Crouse’s assessment of this:
It appears to me that the 7th edition of the MLA Handbook for Writers of Research Papers in § 5.6.1 comes very close to saying, “It’s out there somewhere; I found it; you probably can, too.” … Many of [their] points are well taken. But I would urge that you always give the URLs that you used to reach the cited material. Why not give your reader all the help you can? Why make him or her do a search for a source for every item in your paper? If the RL fails, then he or she can always resort to the searching that MLA recommends.
All in all, I am very impressed with Crouse’s recommendations in Citing Electronic Information in History Papers. If you are looking for some sensible advice, you might want to start there.