Let's talk profanity. :-)
In my students' Creative Writing papers, I allow a certain amount of profanity. I tell my students that if their characters really need to cuss - if that's part of who they are as characters - then, let them.
But, I also caution my students - too much profanity can actually detract from the writing. Case in point, the movie Good Will Hunting. Now, I like that film. It's got some great moments in it and some well-written sections. But the first time I saw it, all I could hear was the "f" word. I even started counting them, for fun, and lost track after about 30 or so. I'm not a prude, and I don't mind profanity in movies, to a degree. But when it overshadows the dialogue, the characters, the plot, it's too much.
Think of it this way - if you have a character who uses that "f" word every other sentence, then after awhile, the reader starts to become numb to it. The word loses its impact, just like any other word you might say over and over and over again. But, if you have a character who never, ever cusses, and suddenly that character is infuriated by some situation and says, "F$%# that!", then it makes the reader sit up and pay attention. It's much more effective, used in moderation.
Certainly, profanity is not mandatory. In fact, most of my characters don't use profanity, and when they do, it's in moderation. That's just my personal choice, and it reflects the kind of novels I write ("cozy" women's fiction). Action/mystery seems to be a genre that lends itself to more profanity, but not always. I once heard John Grisham say that he uses very little profanity (if any) in his books, and that certainly hasn't stopped the sale of them!
In the end, it's your choice as a writer to include profanity or not to include it. Just be sure that it's necessary to the character/plot, and not simply "thrown in" for the sake of being controversial or startling. Every word, even cuss words, must serve a purpose in your writing.