I was able to find my 1960-2012 data, and according to that, the worst 25-year period since 1960 for the total market has been +8.37%, for the period from 1960-1984. The returns across 1929, though, were much worse. So I agree, the best odds are in the market. That's not to say you can count on those returns as a sure thing.
I just want to reiterate a couple of important points, and both are related to what I think is the scenario we are discussing here. That is, when considering your choice of either buying
DVC points with a lump sum payment, OR, setting that money aside in a particular investment, and spending it annually (or however often you go on vacation). If there is another scenario, then we are talking about something different, which is fine, but assuming this scenario, keep in mind the following:
1) We are talking about an initial investment at a point in time. I don't know if those rolling periods are done on a daily basis, or if they take an average of the stock market value during the year, or a specific date in the year, or whatever. But unless they are going on a daily basis (so there would be ~250 data points per year, not one), then it doesn't effectively capture the range of possibilities. If you start your investment on the highest market day of the year, your returns at any time point later are going to be worse than if you happened to invest from the lowest price point of the year. Also, this scenario does not include periodic investment or dollar cost averaging, which helps smooth things out.
2) Because the money will be used on an annual (or whatever) basis in the near term for vacations, a simple long-term value (whether that is 15, 25, or 40 years) is not appropriate. You are going to be withdrawing at least a portion of that money over the short term, and if the market does poorly in the near term, you will never achieve the long-term return hoped for because you are withdrawing your base.
This again is just for the scenario of comparing buying DVC points vs. saving/investing that money and using it annually to pay for vacations. If you are thinking of buying DVC as part of an investment portfolio (which I explicitly do NOT advise), then comparing long-term rates of return makes perfect sense.