They are not useful at all, or else when we first had permissionless distributed databases in the 70s we'd freaking keep using them to this day. Regular databases like we have now are more robust and reliable, don't eat up a ton of energy like distributed permissionless databases (Bitcoin eats up an estimated 34TWh per year and growing as of 2017, there's no other database on the planet that consumes so much yet does so little) and regular databases don't need fucktons of environmentally-wasteful hardware to operate.

We figured this out in the 70s. That some idiot resurrected the idea in the '00s and got a bunch of people to buy into it tells much about the state of "technology enthusiasts" on the internet.