> A decentralized digital currency where the proof-of-work has a second utility having a great commercial value (such as folding proteins) would be quite frankly revolutionary and would have a good chance at displacing Bitcoin.

Why?

Let's say someone were to solve the protein folding 20 years from now. It would no longer be a challenge. This would mean the crypto-currency that used the once-hard problem of protein folding would no longer work, as the mining problem would no longer be difficult.

I think a crypto-currency is much better off doing something useless, as the worst thing that can happen to a crypto-currency is mining no longer being difficult. We want mining to solve the most difficult problem there is: breaking hash functions. Not solving something that has a greater chance of being solved, because it would mean the end of the crypto-currency.

We need something that we think and hope will stay difficult for as long as possible. Hopefully, protein folding will not stay difficult, because there are immense advantages to it being easy.