I purchased pre-construction in Las Vegas. This is how it works. You get an purchase incentive in the form of points when you purchase. You can use the points for hotel stays. You will be told when your use year begins. Let's say for conversation purposes it is 2010.
You do not have to pay any maintenance fees until you can use the resort, and if you live in New York you don't have to close escrow and pay any additional money until the building has a certificate of occupancy. NY'ers only have to put down 25%. You are usually given the option of purchasing the "alt use" points (alternate use) if you want because you don't have anything to trade and you don't have anything to occupy. The amount of points are what your week would trade for if you turned it in. Different weeks are different amounts. If you had a week that was worth 110,000 points then all you have to do is pay the alt use fee and Marriott will give you 110,000 points.
You can usually purchase one or two years at purchase. This is the only time you will ever be able to do this - when you are purchasing something that isn't ready to use. If it will be 2 years before you can use the resort you can choose. Let's say you decided to purchase the alt use points 110,000 +110,000 (2 years worth) 150,000 purchase incentive points, 18,750 points for putting your 25% down payment on your Marriott visa and VIOLA! you have 388,750 points which will give you an incredible trip for 4 or 2 incredible trips. - hotel and airfare.
The other plus is you don't have to start paying on this for 2 years and you got the property at the entry level price. Marriott raises the prices of all their property regularly.