I can see the slippery slope argument, however it overlooks the fact that countries/states are already willing to implement the non-privacy systems.

If these systems take off, it will give privacy advocates the ability to point at California's system and say "look, they have a system that is as effective as the strong assurance stuff but without the people sending you angry emails."

I see it as almost a "reverse slippry slope". A way for people to push for less strict verification.