Can someone please explain this concept? I thought the capacity at the resorts is limited by the number of points sold for each resort. Also, if more members choose to go during a specific time period than space allows, that seems like a recipe for DVC to do a point re-balancing at some point in the future. I don't see how it will necessarily get harder to book some sort of vacation, though.
Actually you're correct. The quote you posted isn't really true. The only real impact is if DVC builds more resorts outside of WDW.
Within WDW it's a wash. While competition for various resorts at 7-months may increase, it only means more availability somewhere else. Suppose at 7-months a large number of SSR owners book all the available rooms at BCV. OK, so you can't get into BCV, but that now leaves a vacuum at SSR. All those SSR owners who got into BCV will not be using points at SSR, so you would be able to get into SSR at 7-months.
The only real issue is when non-WDW resort owners book within WDW resorts. If a HH or VB owner user uses their points at a WDW DVC resort, that leaves vacancies at HH or VB. If there isn't an equivalent number of WDW DVC-owners using their points to book back into HH or VB, then an imbalance could occur.
In reality, this is probably a non-issue.
As to buying into HH instead of SSR, for example, the dues will make a subtle difference. If for example HH is $0.70/point more than SSR, and you can save $10/point by buying HH instead of SSR, then in 14 years the extra dues have eaten up all the savings you originally made.
There's also the chance SSR dues won't increase as much as HH dues. As more of the resort is sold out, there are more members to contribute to the 'communal' costs of running the resort. Thus some of those fixed costs are shared by more members. Dues could even go down once or twice (This actually happened at OKW as the resort sold out. Dues went down as more members joined and fixed costs were spread out over more people. After it completely sold out, dues then started going up again, basically at the rate of inflation.
Finally, albeit a very small chance, if you purchased HH, and it was comnpletely destroyed by a hurricane, you would not be able to use your points at any other DVC resort. (Because the other DVC resort owners would not have the ability to book at HH, thus no 'exchange' is possible). HH owners would probably get some sort of insurance benefit for their loss, and the HH resort would be removed from the DVC 'family' of resorts.