Josh Olson, the writer who did the screenplay for A History of Violence, wrote an article for the Village Voice entitled I Will Not Read Your Fucking Script, explaining exactly why he, and many other pro writers, won't read scripts, stories, novels, outlines, treatments, etc., that hopeful newbies try to hand them. Although his tone is rather harsh [cough] he makes some excellent points and I agree with him; pro writers don't owe random newbies anything. If they're asked by a random newbie (or even a newbie with a vague connection, like a spouse's brother's roommate or similar) to read a story -- or recommend the newbie to their agent, or share names/numbers/e-mails for editors, or whatever -- then "Sorry, no," is never a rude response and doesn't merit any immediate abuse or later bad-mouthing to others.
There've been some interesting responses from around the net, and Cleolinda over on LJ has the best collection I've found, along with some personal input of her own. She's a published writer herself, and has had relevant experience.
The original piece and some of the responses focused on obligation and courtesy and favors, and whether or not a pro owes anything to random newbies. Some of the other commenters point out that there are also legal issues involved, and that pro writers can be and have been sued for plagiarism because they read (or could have read, whether they did or not) some newbie's story or idea, and later came up with something on their own which the newbie thought was too similar. See David Gerrold's link in Cleolinda's piece, in particular, for an excellent take on that side of the question.
This issue affects every writer, both published and hopeful, and I recommend everyone read this set of posts.