Remember that it only counts as "saved" money if you were going to spend it anyway.
For us it is "saving" us nothing, and in fact I believe it will cost us. Without it, we'd go every three to four years instead of every one to two years. We wouldn't be paying for airfare, park tickets, t-shirts, greens fees, or meals out nearly as often (we don't live nearly close enough to drive, and we aren't the type to save money by cooking our meals back in the room, and we always come home with stuff). We'd be perfectly happy for a number of years in a single hotel room - and eventually we'd need two, but only for one or two trips when the kids are teenagers.
But, we can afford a DVC membership (or an annual or biannual Disney trip) and would like to take one. We are now more likely to go more often. We are more likely to stay in a room where we adults are not tripping over the kids. I can pull my workaholic hubby away from his office because "we have to use the points."
I suspect, knowing myself and my husband, that even though I know we go more often the psychology of the trip will be that each trip is cheaper, and I'll be more likely to spend more money on more frequent trips (I also suspect that's Disney's secret hidden agenda behind the DVC).
I also think this is my family's situation, and that some people do "save" money with a DVC membership. At least one person here cooks almost all meals in the room. People who live within driving distance aren't adding huge transportation costs. Anyone who already goes yearly, staying in a Deluxe (and is going to keep doing so) will save money.