Product

Why do people think proof of human is useful?

Because the entire world operates on the fact that we issue identities for real persons for various practical purposes? Passports, drivers licenses, taxes, school, etc. 

Makes it easy to assure that any services are not Sybil attacked.

Eng @superstate. Chasing curiosity. People, nature, machines. Hakuna matata, "Its a problem-free philosophy.”

Ethereum

The inverse of not trying to have PoH means free for all and the lack of an ability to solve certain problems (UBI is one example, but all sorts of welfare programs that are crucial today).

Much of this would be true today wrt to Medicare, Medicaid, SS benefits, and so on. 

It would be important that governments / (insert new institutions) penalize abuse via harvesting.

What stops people from harvesting credentials, especially if the expected value from having one increases relative to a normal Ethereum address?