Personally, I'm fine with a bit of swearing. I swear on occasion (probably more than I should) and I don't have a single friend who doesn't swear. Quite honestly, it's part of being a teenager, and I'd wager that 95% of teens have sworn at one point or another. There will always be exceptions to such a generalization, but for the most part swear words are consistently present in day-to-day high school life.
Here's the thing: as an author, you write from your main character's perspective. What your main character chooses to do and say does NOT necessarily reflect your own beliefs. For instance, my main character steals a human child every year for a ritualistic sacrifice. Does this mean I condone sacrificing infants? Of course not. My main character is not me, and I am not my main character. At times, I feel readers (parents in particular) have a hard time separating the two.
If you write a book from the perspective of a girl who's dabbled in drugs, odds are she swears. Sanitizing all her language will reduce the authenticity of your work. Trust me, readers will notice. On the other hand, forcing a character who otherwise wouldn't swear to use foul language (usually in an attempt to connect with a teenaged audience) will feel just that: forced. Immerse yourself in your main character's thoughts. What would she/he say? Write from your character's perspective, regardless of your own moral values. A writer who lets his or her own beliefs overtake the organic actions of a character is compromising the impact of the project.
On a forum the other day, one of the commenters said something along the lines of, "There are always ways to avoid swearing. Authors only swear when they can't come up with a more creative/intelligent way to express feelings." Now, could an author come up with a more creative and less vulgar way to say "fuck you?" Certainly. But would the character in question purposely abstain from using the word "fuck" in order to avoid foul language? Doubtful, in the case of most teenagers. Any attempt to censor language at the expense of a character's individual voice does a disservice to the author and the reader.
I've also heard parents express concern that teens will "pick up on" swearing in YA, and thus use the words themselves. This may be true in select cases, particularly younger/less mature teens, and I think it's appropriate that some parents help select what their children read. But by the time teens enter high school, they're surrounded by swear words all day. Anybody who says otherwise is kidding themselves. Considering the amount of foul language teens hear regardless of what they read, swearing in a YA novel is not going to change whether or not they swear. Most kids are smarter and more self-assured than adults give them credit for.
Swearing must be deliberate. It must be an accurate representation of the character, and the situation should warrant such an expression of intense emotion. I think it's entirely possible to write a clean YA novel with an authentic ring. However, in my opinion, there is most definitely a place for swearing in many YA stories.