We just got back from 5 days on Manihi where there was no meal plan included with our package.
We went ahead and got the half board, and really we would have gotten off cheaper if we had just paid a la carte.
The price of the meal plan assumes that you are going to have a 3 course meal every night and pick the most expensive items.
There were a number of nights when we did not feel like dessert or a starter, and had we been paying a la carte our dinners would have been 25-50% cheaper.
Also, we like to eat fish (it doesn't get fresher than this) and those tend to be the cheapest options... you pay more for steak that is flown in from a thousand miles away.