You've gotta think carefully about smaller "extension" contracts. Adding on to your home resort, no problem. But if you're buying a small amount of points at a different resort, you can't assume you'll be able to combine those points together for one vacation. You may be able to do that, depending on where you want to stay and when you're going (for example, the larger resorts - OKW, SSR and VAK - will usually be easier to book at the 7-month-or-less window, which I'm sure most of us already know). But if that doesn't work, you're dealing with always splitting your stay between two resorts - one or two nights at the resort where you have fewer points, followed by the rest of your stay.
What I'd recommend is what other people have suggested - if you're buying 25-50 points in a second resort, try to always bank and borrow those points so you can stay at that home resort for a longer trip every three years.