I do include characters who curse in some of my stories. Some people curse in real life, and I want my characters to feel real.
People curse for various reasons, but the most important (to a writer) is that doing so reveals their passions and stress points.
All forms of cursing are means of expressing extreme emotions, from fury to sadness to unbridled happiness. Not every person does this - but those who do are releasing energy from inside themselves that needs to be released. Most often, that pent up energy is stress. People under extra levels of stress are more likely to feel extreme emotions at the slightest provocation. And that stress can come from anything. It can be poverty or critical life events or social pressures (both national and personal) and we can't forget mental health issues. I've known people from so many different background that both do and don't curse, that I absolutely defy anyone who says it's any kind of "moral" or "social status" thing. A poor person. may curse or may not, depending on how the rest of life is going. A wealthy person may curse or not, depending on how their life is going. And people raised not to curse may adopt it and those raised to curse may eventually stop. And every fluctuation in between.
People who are offended by cursing may be summed up as being offended at someone else's stress. On the one had, this can be a nurtured thing—as some people are forced to "keep it all inside". Others may have there own mental health challenges that can't cope with someone else's outbursts. Regardless of its root, however, condemning a person for cursing is demanding a victim to suffer their pain in silence. And that is abuse, pure and simple.
When I was young, I couldn't handle people cursing around me. I had a great deal of stress inside me and my parents were critical of cursing. And when people cursed around me, I felt outrage... At least, that's what I thought it was. In reality, I was jealous. Why were they allowed to cry out in pain and I wasn't? How come their problems and passions were validated and mine weren't? Well, it turned out that my parents had been raised to suffer in silence, as well. It's a cycle of abuse that needs to end. And when I finally decided to curse (I don't do it all the time), it helped. It really did. When life hurts or inspires great joy, a human being is allowed to express themselves in as extreme a manner as they feel. When my children curse, I take notice and talk to them—not to criticize—to find out what's going on in their lives that inspires those expressions. And if it's because they are happy, I let them. If it's a really fucking good book, they should enjoy it. But if they are unhappy and angry or fearful or pained, that's what I'm there for to fix (as best as I can).
I don't write a ton of characters all swearing at every possible moment to form a milieu of abuse. That's not me. I use it to reveal how individual characters are suffering and what they are passionate about.
In The True Dragon of Atlanta, May curses for many reasons. She is under extreme stress. That stress comes from her station in society—used and abused by the general public. Hated by those who don't want her to exist and pushed away by her very own clients. She is lonely and ignored and screaming into the void just to hear her own voice. And her little sister June is the most extreme curser of all. She does it because she is constantly crying out for attention—her sister's attention, who already curses. Almost no other characters curse. And in the end, May stops cursing. It's subtle and no lantern is held to it, but the change in her is palpable. She doesn't need to, anymore. Oh, she still feels great things, but she's no longer unheard.