I'll be the first to admit that the technology can be abused-- that it's even ripe for abuse. That sort of problem can be avoided by allowing 'enough'-- and if the goal is to just prevent a site being flooded out 'enough' could be pretty high.

Of course, I think the effective purpose of google's attest feature is to invade everyone's privacy which we should assume is part of why they don't use privacy preserving techniques. Privacy preserving techniques could still be abused, however.

Maybe they're even worse for humanity because they make bad schemes more palatable. I think right now I lean towards no: the public in general will currently tolerate the most invasive forms of these systems, so our issue isn't that they're being successfully resisted and the resistance might be diminished by a scheme which is still bad but less bad.