You are pretty much guaranteed a cabana if you are concierge. I went on at 120 days and grabbed one very easily. Also, a friend of ours sailing for the first time didn't book anything right away, called shoreside concierge a few weeks ago and got a cabana on the family beach. They seem to hold some back for concierge requests.
Along the same lines of the cabana, you have a more stress free time while planning. If you arrive and come to realize that your bookings conflict with something else you want to do, or want to change your dining, spa, etc, you just tell the concierge hosts that first day and they can take care of it for you. 5 sailings for us and not a single disappointment. They act as your liaison to guest services. You never have to go to or call GS or go find someplace to make changes, wait on lines, put names on wait lists, etc.
One warning though. It is an expensive habit that I cannot break!