Are job descriptions a worthwhile tool for organizations these days or are they too limited?

PLEASE ANSWER THE FOLLOWING AFTER WATCHING VIDEO AND READING ATTACHED DOCUMENTS: Are job descriptions a worthwhile tool for organizations these days or are they too limited?  Have jobs become harder to define as they’ve become more multi-focused?  How do we find the right fit between what the requirements of the work truly are and what the candidates say about themselves?  How can the very definition of the job help us find the right people?
State your observations about how we can arrive at a good, workable job description and how we can use that to attract and find the right candidates.  Should a job description be very specific or should it be more open-ended so that we don’t miss some good candidates?  Find sources that support your views.
In your experience with on-line job sites, what good things have you seen that make it easier for a potential candidate to use?  What difficulties have you seen in using these sites to look for a job?  What piece of advice might you give to a company person who wants to post a job on-line to find people?