We bought because we realized just how much we were spending a year... we always stay at deluxe resorts and we were spending a lot of money. This was not a problem when we were going every three or four years, but now that DS is 3 and really appreciates WDW, we have started going at least once a year and sometimes more. We went in September last year (stayed at the BW) and May this year (stayed at the GF) and had starting planning a trip for this fall...
We'd been thinking about buying DVC sometime in the future. We have a lot of expenses right now - (it looked like we'd be paying for IVF, now we're about to be paying for adoption expenses) - so we kept putting it off. Then we thought about how much we'd be paying for our fall vacation, and decided to go ahead and buy in now. We'll get two vacations next year on our current points, and we're planning to buy more points when our expenses are more clearly defined.
We would have spent a minimum of $3000 for accommodations for our next trip since we planned a family trip with the three of us, plus my mom and her SO. Assuming two trips between now and the end of 2010, we were looking at $6000. We can do that with our developer points and first year's points with minimal or no borrowing. (We may actually be able to do three trips if we stay in a studio.)
Anyway, after crunching the numbers, we decided that it really did make sense for us. People talk about it breaking even for them after 8 or 10 years, but it will be sooner for us based on our traveling habits (and the nice bonus of developer points helped make the number look good fast.)