I just booked Nachi for my next cruise. I don't believe there is any option other than All Inclusive, but the amount of money seems to be reasonable for what you get: more than enough to eat, an open bar, and guaranteed lounges/umbrella on the beach (they limit the resort to 100 people per day).
The reason I chose to actually make a reservation is that there will be six cruise ships in port on the day that we are there. I decided Paradise Beach (or some of the others) had the potential to be too crowded for our liking. Also, with six ships in port, I was concerned that if we didn't have a reservation, Nachi would fill up and we wouldn't be able to get in. It's a calculated gamble on my part. Lastly, I've seen nothing but glowing reviews for Nachi on Trip Advisor and Cruise Critic, so I figured we have nothing to lose.