If you use a money with an issuance scheme that doesn't derive from pure costs (useless things like hashing), the underlying incentives of that money will seed its own destruction.

What happens if you have a coin based on "useful work" and you "solve" the problem you were hoping to solve. Now a lot of "miners" leave the network, presumably because they were only there to help solve that particular problem. Now you have a weak system, and a system subject to 51% attacks by adversarial miners, and therefore not a good money/coin.

I don't see how HN commenters (in large part) don't get this. It's the Single-responsibility Principle, just in a different context.