Have you ever considered the possibility that print shops, email sites, the WGA/WGC, coverage services, et cetera, even the copyright office, steal non-copyrightable ideas? I recently joined Zoetrope Virtual Studio and realized that people just put their (albeit bad) screenplays up for everybody to read. If I wanted to, I could write ripoffs of these unproduced screenplays and get off scot-free. Likewise, agents can share ideas with their (other) clients, managers (especially the ones who are producers) can just steal stuff. Hell, what if an agent or a development executive or producer is also a screenwriter? It's not unheard of for production companies to do this, but what about other parties? All you need is a logline. Dinosaurs fight Nazis on the moon. Now you can write that screenplay and I can't complain about it.
I don't think print shops steal scripts. (What's a print shop, Grandpa?) I would be stunned to hear that the WGC or WGA registration services, or, Lord knows, the Library of Congress steal scripts. I would be stunned to hear they even read them. And there's no percentage in an agent stealing a script when he could just offer to represent it; and a producer won't steal a script when the writer would probably be only too happy to option it for a buck.
But writers? Especially non pro writers?
I have often wondered about sites like Zoetrope, where bazillions of nonpro writers post their loglines, and read other people's loglines. If you've got a really great hook, the only thing keeping other writers from stealing your hook is the knowledge that you've already written the script, so they're at least six months behind you. Is that enough? Usually it is. But if they have access to your script and find out that you've done, in their opinion, a really terrible job delivering the goods on our hook, they could poach it.
When I was teaching a writing seminar, I came across a script with a brilliant title and hook, but which I felt didn't deliver the goods on the concept. At all.
I'm not a thief, so I optioned the script and got some producers to hire me to rewrite it. But what if I had read the script on an online forum where there was no evidence that I'd read it? What if I dumped the title and wrote another script, with an entirely different plot that, I felt, delivered the goods? Legally, I would be free and clear. You can't copyright a hook.
Later on, the guy refused to re-option the script, so while I got paid for my rewrite, the project is dead. So I paid a price for being honest.
I've never posted my loglines on a site. And I recommend you pitch your script idea to anyone but fellow writers. It's too easy for a writer to "forget" where they got the idea.
However, I think non-pro writers generally have much more anxiety about their ideas being poached than pro writers. If you have a really great idea, odds are, no one is going to steal it; you're going to have to ram it down people's throats. It's all very well and good to think up "snakes! on a plane!" But then you have to deliver the goods. That is, as we say in computer science, non-trivial. A great hook is worth money, but only if you figure out how to deliver the goods on it.
What I was suggesting about agents is that maybe one of them represents Joe Bigshot and Joe Schmo sends the agent a really great hook. If it's the agent's policy to reject any and all queries from schmos, then the agent can tell Joe Bigshot the hook and have him write it.
I am pretty sure this doesn't happen. First of all, you won't be submitting your script to Joe Bigshot's agent; or if you do, he won't read it. He doesn't want to rep a "baby writer." Secondly, Joe Bigshot doesn't want to write your script. Assuming he's not busy with paid assignments, he's got a backlog of ideas he thinks are wonderful. It would be rare indeed to find a pro writer who not only would be willing to steal an idea, but who also likes your idea better than his own.