My general take on it is that sex, violence and swearing are completely unnecessary - except when they're essential.

(Which applies to everything in a script, so is probably a completely pointless comment.)

Totally agree that it depends on how you are approaching the story. If you are using them solely because you can't think of better or more appropriate dialogue (and why is spell check so anti-dialogue? I don't want to write dialog. it's so ugly), then it's probably a problem.
But if you are writing a raunchy comedy, or some gritty drama or anything you know will pretty much be R, do the damn thing. But I just started a new action-spec right after a college comedy I recently finished (torn up with offensive expletives), and am purposely staying away from the cursing with a PG-13 in mind.
HOWEVER, a question I do have, is what about using language in the descriptions of the story that will never see the light of day? Is that frowned upon if it doesn't match up with the aimed-at rating?
I mean I know most of Shane Black's stuff is rated R, but he is damn effective at enhancing his descriptions with some well-placed f-bombs and profanity. opinions?

Isn't it very easy to change all that should you be lucky enough to sell the thing? Take "Easy A" for instance. That was extremely vulgar but they changed up the dialogue to get a PG-13 and the movie still resembled the script and made a great movie.

On the topic of the rating board it is interesting to see
"This Film Is Not Yet Rated"
http://www.imdb.com/title/tt0493459/

@Danny: I've seen writers use profanity in scene description. Sometimes it works. Other times it's really obnoxious. First you have to make sure that using it is in sync with your Narrative Voice. If not, it will stick out like a sore thumb. Second just know you are taking a real risk with readers as there were so many Shane Black imitators, the industry got pretty much sick of the whole breaking the fourth wall scene description convention ala The Last Boy Scout spec.

But if you think it really works, then by all means go ahead and try it. If your story is great, it won't make a difference if the script sells or not.

There's two different issues with profanity.

1) Profanity in dialogue.
2) Profanity in scene description.

The first one pisses off the MPAA. But it's really the second one that some script readers are Nazis about. Yes, Nazis.

LOST scripts use a lot of profanity in scene description. I like it. Some people don't.

I had a spec feature that had 0 profanity in the dialogue and a few minor swears like shit and damn in the scene description. The prod co loved the script.

However, one of the notes I received was the swearing would have to be toned down if they were to shoot it. This was a Christian group.

I was floored -- none of the swearing would even be on screen. But still -- there was the note.

I'm very pro swearing in scene description, but I will throw this out there...

In TV, especially cop shows, and edgy content you can swear to your heart's delight in scene description.

In spec features, you may need to be slightly more judicious. You may have a Christian group reading your rollicking adventure script.

Ultimately though, write for who you think will like the film in the first place. Don't change your voice because one person is offended.