"Since patents in this scheme cannot be used to attack other participants, they will be limited to defensive uses only."
More precisely, if your description is correct, they can be used to attack -- just not other DPL participants. In fact, the best case here would be that the DPL pool gets large enough that its participants can continually harass bigger players with semi-frivolous patent suits. Ideally, you'd set it up so that any DPL member can sue any non-DPL member for infringement of *any* DPL member's patent. Invite the patent trolls on board, and give them boatloads of patents to attack companies with. The victims would be forced to either shell out lots of money -- or join the DPL themselves. Once you get enough patents, and enough lawsuits flying around, even the biggest players might have to concede and join.