The market price of each certificate will be the function of the underlying asset and the trustworthyness of the issuer.
So, all the downsides of both currency and barter, and the upsides of neither. Like I said, look into Ripple.
Yes why not cap the limit at 21 million, pretend we got intrinsic value and reward early adaptors for selling to greater fools in the ponzi scheme.
After all it's safer to pay 1000$ for 1/21 millionth of a ponzi, then to pay 0$ for a blank sheet of paper that could be used to sign a contract for gold, stocks or real estate.