Hey TF - don't know if you remember our debate over PMB's, but Deprived/twentyseventy's derivative model solves the perpetually-yield-less issue by adding a shorting mechanism. Of course, before ASICs came along, PMB's were actually not the worst investment out there.
As always, how good an investment (like a PMB) is depends on the price. Prior to DMS-like mining derivatives, asset issuers created PMBs at pricepoints where it is extremely unrealistic for buyers to make a profit. The price is what made those PMBs bad investments.