I think too much profanity can ruin any good book, even for adult audiences. It's true that many people swear when they talk, whether they're women in a factory, gansters in a warehouse or adolescents on a street corner, but they also "hum" and "err" and fluff words, talk over each other, forget what they were saying, repeat themselves and mumble.
Dialogue is different to speech.
Dialogue is not a facsimile of actual speech, otherwise is would be a pain to wade through. Using swear words simply because you think it gives it a touch of realism is a mistake, because to give the characters justice you would have to splatter the page with obscenity and soon the words loose their power. In general speech, profanity is mainly used as a sort of wild card adjective, and almost always means "very". If you strip it out you lose nothing, and emotions, even anger, can be conveyed in so many better ways than having a character swear.
But, there is a place for it, especially if you have a character swearing in an unusual place, say in church, or accidentally at a family dinner. It can be used as a comic tool, rebellion, immaturity and a load more, but if you over use it, the power is gone.
Example - who really notices that Eminem "cusses" in his songs - yeah they work, but it doesn't raise any eyebrows. Whereas is George Bush said something in a speech to the nation, "These f*ckers have gone too damn far" - we'd be talking about it for years.
I've allowed myself a few minor swear words and one "F*ck" in my YA novel and so far the agent hasn't mentioned it, but to put it in context, the novel is incredibly violent, involving cage-fighting and gansters where you'd expect a mass-swear-a-thon, but what's the point?
Grey