The swearing? Well, consider the characters - pimps, whores, drunks and the lowest form of mankind. With out cursing they would have no adjectives to express themselves. :O)
I agree. The show has gotten some criticism over the abundance of profanity, but it is depicting an Old West mining camp, and an illegal one at that. It was a place that was filled with rough characters and the scum and dregs of society, and those kinds of people were notoriously profane. I see the profanity as adding to Deadwood's realism.