Think of bitcoins as tokens or as tulip bulbs - it can be used for something - but the prices is not the main idea. If you buy them for the price rise you are creating your own "ponzi scheme"
> bitcoin as fixed amount of currency theory is a bit questionable.
The total amount ot bitcoins is not questionable.
> There is no reason why a fractional reserve bitcoin bank could not pop up and create bitcoin-as-payment-money out of thin air
If a bank lend out x times the value of the bitcoin deposited, they will not be able to pay them out as bitcoins.
If you dont have access to your bitcoins as bitcoins, but only as a bank statement about how much they owe you, you effectivly traded your bitcoins to an IOU.