Okay, so here's my unqualified unprofessional unreliable hypothesis on release forms. If you don't sign it, your screenplay won't get read. It's pretty much that simple.
That's not to say you should sign a release given to you by any ol' toothless dude in a mobile home with "production company" spray painted over the door or some guy sitting in the Walmart parking lot accepting submissions from the tailgate of his El Camino. But, come on. You've signed more complicated stuff. You've probably given away a kidney or two on your credit card applications and do you even know what you agreed to when you refinanced?
If a legitimate agency, production company, or manager gives you a release and you want them to take you seriously, sign it. Yeah, we all have to start some place and maybe you have to work with some small company that really is in a mobile home. Fine. Take the release to an entertainment attorney and/or research the company's history with other writers.
Friends, a release is standard operating procedure. You have to sign one so your kid can ride a bus to school, you have to sign one before a doctor will look at your hang nail, and you have to sign one before a production company will read the work of an unknown. Whether established screenwriters sign them or not is something you'll have to ask a pro.