alt Hacker News> They aren't useful outside of "financial engineering."
Without disagreeing with your overall point in 99% of cases, we did actually have a good use for pinning things in the Bitcoin blockchain when I worked at Keybase. If you're trying to do peer-to-peer security, and you want to prove not only that the evil server hasn't forged anything (which you do with signatures) but also that it hasn't deleted anything legitimate, "throw a hash in the blockchain" really is the Right Way to solve that problem.
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The property that makes the blockchain useful for this, though, is that it's widely-distributed. "Throw a classified in the national newspaper" is just as good. Nowadays, we have better solutions (appendable BitTorrent comes to mind), with most of the advantages of blockchain but few of the disadvantages.
> If you're trying to do peer-to-peer security, and you want to prove not only that the evil server hasn't forged anything (which you do with signatures) but also that it hasn't deleted anything legitimate, "throw a hash in the blockchain" really is the Right Way to solve that problem.
and it only requires the same electricity as a medium sized country to do it
continuously, forever