I don't think the cruise lines want to start a trend of too many people choosing to walk their own luggage off the ship, but the truth is that the traditional disembarkation procedure is a pain. The normal method is to take all the large bags off the ship the night before and then to call passengers off the ship according to color codes (for groups of bags placed together outside) to retrieve their bags and continue off the dock.
The cruise lines instituted taking hte bags off the night before because they do not have the personnel to help every passenger take their bags off in the morning. The color code system prevents chaos in the terminal as people look for their bags and walk through customs. They hand out the tags in the order people require for whatever plans they have that day - the ones with early flights, for example, getting tag colors that get called earliest in the morning. What this does is eliminate the need for porters for all the passengers leaving the ship at the same time.
NCL has officially instituted free-style disembarkation where one is allowed to carry one's own bags off and leave at any time. And the truth is that if you do not have color coded bag onshore you can pretty much leave any ship at any time.
I believe I can say that your statement is true as long as you only have one or two small bags and do not require a porter to help you. I should caution you that it is not official policy, however, and if you try it on a non-NCL ship with a large number of bags, do not count on having anyone from the cruise line there to help you carry the bags, and do expect to have navigate through a lot of passengers and turmoil on the way out, which is not easy with a lot of large bags. Also expect some dirty looks from people waiting to get off and a long trek down the gangway with no assistance.
If you have a lot of bags, and require an early disembarkation, you can always simply tell the cruise lines you need to be one of the first people off and they will give the proper tag. Also - they ask to put all your bags out, but the truth is if you want to leave one medium-sized one behind to pack your jammies and bathroom stuff there is no problem with that.