See:
http://touringplans.com/walt-disney-world/hotels/number-rooms
As I indicated in a previous post, there now is the equivalent of over 4000 DVC rooms. Per the above link, WDW has about 23,700 non-DVC rooms (excluding Shades of Green, Dolphin, and Swan which are not run by Disney). Considering that DVCs run at about 98% occupancy while Disney's other resorts run at about 80%, DVC members now represent as many as 20% of WDW onsite guests. That's potentially 20% of rooms being paid for at cost. Some DVC rooms can be combined so the actual number of DVC guests per day is almost certainly lower but, hopefully, this makes it clearer that DVCs now are becoming an important factor impacting margins.
Guessing why the Poly's next, perhaps there's not enough room to add at DLR, DLP is not considered a good location to add DVCs, while a TDL DVC could hurt
Aulani sales. What's clear, at least to me, is that VGF and Poly should sell very quickly. They seem like low-risk/high-return choices.