My claim is that a deflation caused by an increase in hoarding is not equivalent to that caused purely by economic growth and increased efficiency. Deflation caused by an increase in hoarding (a decrease in the velocity of money) is not harmless.
Sure, that would be relevant if some government forced people to hoard. However, ask yourself this -- why would there be an increase in hoarding? And what's the right reaction to that? (Hint: It's an increase in hoarding.)
Ideally the velocity of money should remain more or less constant and demurrage is the only thing that I know that can produce that desired result.
Same thought exercise: Why would the velocity of money go up? What's the right reaction to that? (Hint: It's an increase in the velocity of money.)
Sure, if you imagine a health, stable, productive economy suddenly has some magic change in some parameter for no reason, the effect will likely be bad. That's the problem with government interference in a free economy. But it's not a problem when people react to real changes with sensible responses.
The "store of value" / "medium of exchange" balance for money is like the supply / demand balance of prices. It pushes consumption and production to optimum times just as prices push production and consumption to optimum items. Sometimes, the optimum time to produce is now, the optimum time to consume is later, and the optimum time to invest is later. When this happens in a sane monetary system, deflation encourages hoarding.