Oops! Think I may have misread your question (the answer below is based on what I thought you were asking--whether you'd get incentives for purchasing points if/when you add on)...
You do "buy" the points...so you pay the purchase price (with or without incentives) AND the annual maintenance fees...
For example, if you decide to add on 100 points, you will pay 100 times whatever the price per point is at the time you decide to buy...
Hope that helps!
And if you were asking about incentives, below is my original response:
Disney's incentives vary...usually they're based on the number of points you buy (whether as a new purchase or as an add-on), and usually are limited to the resorts they're actively selling. They also come and go. I've seen a dozen or more different incentive programs in my six years of ownership...
I've never seen an incentive for points at a sold-out resort (although you definitely can buy points thru Disney at all sold-out resorts--sometimes you have to wait for the points to become available, but you can always buy them). Nor have I seen incentives for purchases of less than 100 points (and most often it's for purchases of at least 150-160, with the incentives getting bigger as the number of points purchased gets bigger).
But that doesn't mean Disney won't offer incentives for smaller number-of-points purchases or at sold-out resorts, just that I've never seen them do it so far...
Remember: incentives are just that: they're designed to persuade you to buy...if there's enough demand for small-point contracts and/or for points at sold-out resorts, there's no real need to offer incentives...
Do "practice" becoming resigned to adding on (I bought 150 points in 2006, and have added on five times since!)...it really does take a while to figure out what your pattern of use will be--especially since it changes once you have
DVC (at least for many folks, including me!)...
And also look into resale--there are good reasons (for some people, including me) for buying direct (but they do not include getting any kind of "bargain"), but you can save a ton of money getting your add-on points on the resale market--if you're willing to give up certain things (like, for example, using resale points for Disney cruises; and/or the resale process takes a MUCH longer time and is considerably less easy...but it can save you a whole lot of money...so it's worth looking into)...
Good luck!