Clarity is the most important aspect of writing, and no where is clarity more important than in dialogue. Your readers desperately want to be hip to the words that are spoken by each character.
Even if a 20-year-old, drug addict writes dialogue filled with slang, a 60 year old, Baptist preacher should be able to follow the dialogue. If not, that drug addict writer is going to have a very small audience, indeed.
For instance, I was reading a short story the other day that was written by an Aussie friend. He wrote this:
“Dudley,” Cameron said, “put the car tire back into the boot of the car.”
From this sentence it was easy for me to surmise that the word ‘boot’ is Aussie-speak for trunk, as in: “put the tire back in the trunk.” I was thrilled to learn this new word, and I continued to read. Later, I was rewarded by my efforts. Aussies refer to the hood of the car—or some where’s there about—as a ‘bonnet.’