I'm sure there's a stage of Screenwriter Denial that is called, "It's still useful as a writing sample, right?"
That's pretty much the knee-jerk question I'm asked after I tell a writer that their spec isn't commercially viable. Sometimes its because they wrote a drama and nobody's buying dramas. Sometimes it's because they asked me to read an expensive period piece all about Elizabethan furniture movers, and sometimes, it's like last week, when I explained that if another writer has beaten you to the punch on the concept, the script is pretty much dead.
"Sorry man, but your comedy about two guys whose wives give them a week off from the marriage isn't going to sell because Hall Pass just came out. It's WAY too close to it in concept and execution."
-"But it's still usable as a writing sample, right?"
I wrote this response up as a comment last week, but I decided to post it here in the interests of getting more eyes on it and also so that I've got a quick-and-easy post to point similar questioners to in the future. I could have sworn I've addressed "writing samples" somewhere on this blog, before though.
In short, I think writers are a little to quick to grasp that "writing sample" thread when trying to salvage their work.
Best case scenario - You submit an idea that's been done before. The reader gives you the benefit of the doubt and says it shows promise. If your sample is really close to the already-sold or released film, that's a huge benefit of the doubt, because he's taking you at your word that you came up with this material completely independent of the other project and didn't just rip it off. I once saw a writer who ripped off Jurassic Park shamelessly and another writer try to sneak in a Star Wars rip-off as if no one would notice. For a guy like me to take you at your word that this was an original idea of yours is an incredibly lucky break.
Now, since the reader's boss can't do anything with this spec, they ask you to submit something else. They read your second script, love it, decide it can be a movie and sign you.
Worst case scenario - They read your "writing sample", decide you're a hack and the door slams shut. You don't get to move forward with the second script because if a guy like me hated one of your scripts, he's not going to be welcoming to a second one.
Thus, you have everything to lose and nothing to gain by leading with this "writing sample" script because it's still your "real" spec that will made the final decision for them.
I've always been told "agents are looking for something they can sell." The one time I can think that the writing sample might help you is if it proves you're not a "one hit wonder." If you hit them with one awesome and marketable spec, and they want to see what else you've written just to size you up, THEN the writing sample might bail you out.
In the other instances, such as the period drama set in the 1600s, my feeling is that the script isn't going to be very helpful as a writing sample. What good is it to have a strong sample in a particular genre if that genre is more or less radioactive? Sure, you can write all about the Pilgrims, but how does that show you can write a modern day action-film set in contemporary New York?