I've come across this problems few times as a writer and I think there are a number of different issues with foul language.
The first is that although like a lot of people I have no hang ups about swearing and, in fact, swear quite a lot. However, there are lots and lots of people who are really bothered by it. As I'm not writing for myself, but an audience, I have to take into consideration that a lot of swearing is going to define my target market. Not always a bad thing, but it may also restrict the international markets I can sell into and as an independent film maker I have to be aware of different cultural issues around swearing.
More interesting for me around this issue, is the fact that dialogue with lots of swearing is often written to make the piece seem more realistic, more how "real" people talk and the truth of the matter is that it often doesn't achieve that effect, because it doesn't tell us anything new about the character or move the story on.
When it does work, is when you have a character who hasn't sworn throughout the piece, but is driven to it by one defining moment.
The truth is that dialogue isn't about mimicing real life, otherwise we wouldn't go to the cinema, we'd just hang out in cafes listening to other peoples' conversations.