As a writer, I can attest to the fact that you have to be very very careful about dropping profanity, not because it's offensive, but because it can cheapen the dialog.
A single, well-timed f-bomb can ramp up the tension in a dialog sequence, it can show the reader/listener that things have taken a shift in tone, or it can be a hint as to a person's overall character. It can greatly add to a scene, if used correctly.
Used incorrectly, or too frequently, however, it can totally destroy a person's belief in a scene. It's difficult to describe why this works, but I'll try. When we as humans immerse ourselves in a book, or a movie, or a game, it engages a 'higher' function in our minds (Not the technical term, but I don't know the technical term.) We expect things to be more pristine than our every day lives. It's a form of escapism that many of us are not even aware we fall into.
While we're in this bubble of escapism, we notice imperfections, vulgarities, what-have-you more than we do in normal life, because when we're in normal life our brains function in 'mundane' mode. This is why, when most of us read a book where there are characters who -- as they would in real life -- use swearing in lieu of punctuation, it sets our teeth on edge. It doesn't feel 'right', even if the dialog is identical to something you heard on the bus coming home. It's because our minds expect our escapes to be more ideal than the real world, and it's far more jarring to see things like swearing or visceral violence (Like, real-world or SAW-type violence, not those stupid bloodsplosions) in our games.
Now, having stated where I'm coming from, I can answer the question: No, I don't think the level of swearing in DA2 was strictly necessary. I understand why they put it there, to portray certain characters as 'uncultured' or 'rough', but I think they went beyond what was needed. There was a glut of cursing, when all that was needed to get the point across was a few consistent and well-placed vulgarities.