Well, it's not "just" used because of code, it does prevent moisture from ruining your drywall. However, you might "not" need it, but still might be required by building code in your county\city. That's why I said check your local building codes first, if it's required per code than it doesn't matter if you do or don't need it; you have to do it!
Now, don't think if I don't do it they'll never know! If something due to not having a vapor barrier occurs and you need to use your homeowners insurance and an inspector finds out you didn't follow code your policy WON'T cover it. Worst case scenario, but it's better to be safe rather than sorry.