IF you are traveling very light, then call on/off is an option worth considering. But as has been stated by the other, if you has a lot of luggage, don't try it.
Also, another point is for Carnival, ifyou want early "self assist" disembark, they expect you to carry off all your luggage. We accidentally did this on our Conquest cruise. I was busy in the casino the last night and didn't get back to the cabing in time to pack. So, the four of us (my wife and then 12 and 10 year girls had to man-handle 14 bags (my women NEVER travel light) through the long lines and off the ship. The upside is that we were off the ship and on our way home by 9AM. Bad side was the hassle. Luckily, all the bags were wheeled and I was able to strap them together to make it so we could do it.
Would I do it again? No. Not unless it was maybe one or 2 bags each. I sure wouldn't want to try to carry them on. Like was stated earlier, what if you did and your cabin was not ready, I wouldn't want to have to lug them around. You could put them outside you cabin, but what would be the point of carrying them on then? And it you are in the southern ports (which most of the cruises are) do you really want to be pulling luggage around in 90 degree + hight humidity weather?
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013