> a medium of exchange should have stable purchasing power
What's the problem with having a medium of exchange that actually gains value over time ? (which is the case of bitcoin). By the way your USD/Euros and other fiat currencies are actually losing value over time, and not just a little. Look at what you could buy 10 years ago with 100 dollars and today. Is that "constant utility" ? Not really.
"To be widely acceptable, a medium of exchange should have stable purchasing power (Value) and therefore it should possess the following characteristics: value common assets, constant utility, low cost of preservation, transportability, divisibility, high market value in relation to volume and weight, recognisability and resistance to counterfeiting.
http://en.wikipedia.org/wiki/Medium_of_exchange