Read someone's post on Kindleboards.com the other day asking if foul language was necessary in writing...
My first reaction? "Fuck yah!"
Then I thought about it... It can be distracting. Ever watch a C-List comedian fill their 10 minute spot with curse words? Take them out and what's left? One... maybe two jokes?
I have no problem with profanity and hardly anything offends me, but for a character to be a character, they have to have more than one dimension. Too much repetition of any word or style is distracting and can pull the reader out of the story. I'm currently editing a story and found that there are 159 cases of someone "looking" a certain way, "Looking" at someone, or that someone "looked" at something or someone... It made me queasy... The thesaurus is a great invention.
So swear it up, but make your character mean it, and let them have something else to say. Give them depth, even in the gutter.