That is a really good question. Voice catches the attention of the reader, I guess: it gives originality and individuality to a piece that might otherwise be run-of-the-mill. But if concept is (nearly) all in selling your work, then where *does* voice fit? In short, I don't know! Anyone else?

A writer's voice is what sets you apart from the others, makes you different .. in a good way, makes a reader sit up and take notice. It's that little somethin'-somethin' to hang your hat on and call your very own. It's your secret weapon, the flair with which only YOU can tell a particular story in some signature way. It might get you noticed, appreciated, sought .. or, at the very least, not easily forgotten. It certainly seems part of what has people "coming back for more". Any/all of that seems important to me as far as a script's success goes. Or if not "that script", certainly YOU as a writer. I think of it this way --- let's say ten writers are all given the same prompt, the same story idea or brief summary for a desired movie script. Ten writers take the info and each write a screenplay. Let's say half of them "execute" the story in the manner desired. Of those five, let's say three were even steven when it came to dialogue and action/narrative and character development and everything else. It may well be that unique writer's voice that floats one script to the top of the heap over the others. "Voice" may seem like a tough thing to put one's thumb on or to explain, but once you see/read/hear it, you know it. How important is it? Measuring that is like asking me to count the stars. All I know is it's important enough for me to want to carefully develop my own.

I agree with Joni - it's your unique interpretation of the story and how you deliver it to your reader. I believe it can be the magic ingredient that makes the screenplay stand out among others. All things being equal, developing a unique, signature voice is important to make your writing distinctive and stand out - also has your "stamp" on it - not like everyone else.