I'm a patent attorney and I agree.
Unless I'm fundamentally misunderstanding the purpose, there's no legal judo here. It can't protect you against non-participants. It certainly can't protect you against trolls. It only seems to protect you against people/entities already inclined not to use their patents aggressively.
What would be more useful is if it worked this way (it doesn't):
Non-participating entity A sues participating entity B.
Entity B has no useful patents with which to sue Entity A.
Some other participating Entity C does have a useful patent to sue Entity A.
Entity B can use Entity C's patents against Entity A.
But, this scheme doesn't make that happen. In fact, there's basically no way to make Entity C actually use its patents against Entity A on behalf of Entity B. Neither the patent law nor this license could make Entity C's patents a weapon against A.
Even if there were a way, no one with any patents of value would sign up for the DPL since: (1) the cost of enforcing the patent is going to be expensive; (2) turning over the right to enforce the patent to Entity B would be like handing the keys to your car to someone you don't know and assuming responsibility for any damage to your car (e.g., patent is held invalid, found not infringed, etc.); and (3) a priori knowledge of the value of the patent might mean that you'd be in a better defensive position not having granted "free" licenses to every participant (something that inevitably will come up in a damage calculation).
The DPL is a solution searching for a problem. And a foolish solution at that.
A company called RPX has created a much "better" solution for those that can afford to be part of the membership. Of course, in that case, RPX is the point person and has an ability to make things happen and has their own patents that they've acquired.