Find the error in this sentence:
"On July 4, 1776 a printed copy of the Declaration of Independence was presented to the Continental Congress."
Answer: The rules of English grammar dictate that you use a comma after both the day and the year, and the second comma is not optional.
Now this sentence:
"He grew up in Phoeniz, Arizona and still lives there."
Answer: The rules of English grammar dictate that you use a comma after both the city name and state name, and again the second comma is not optional.
These rules are so often abused that when I see someone who correctly uses the second comma, I figure that person probably knows English grammar pretty well. Either that, or s/he is just comma happy.