Well, ok, maybe sometimes.
If your character is meeting up with his buddies for, say, an impromptu vampire slaying session, you can certainly let him exclaim a heartfelt: “Hells yeeeaah, let’s take this goddamn bloodsucker out, whaddayasay!”. By all means, throw in an Aaergheruuhn!!! too while you’re at it. Just do it in dialogue (internal or external), and do it in moderation. Show off a dialect in one sentence, but make the rest of them legible. Having to trudge through an entire scene of dialogue that you have to painstakingly decipher one sentence at a time will most likely annoy the crap out of your readers. Add a touch for flavor, then move on to writing for reading.
Effective writing leads to effective reading, and the goal should be for the reader to read without thinking about the words. The goal is the flow.
On the other end of the scale there is writing words for the beauty of them. I’m talking about those special words that delight you. Those magical words you never, ever get a chance to say out loud. Because, honestly, how often do you use words like wrought, however and succinct when you’re out having coffee with friends? How often do you get the chance to toss in a casual “thus” in a conversation? I know I wouldn’t, without feeling silly. But don’t you sometimes wish you could? There’s something about those words that just resound in me. Evanescence. Henceforth. Cacophony. They have a certain pizzazz to them when you read them, when you hear them in your head. They add another dimension to your writing. But just like with the spoken word, moderation is key. Add some flair, but don’t intimidate your reader.
What’s your favorite word that you never get to say, but love using in your writing?