@garbados you need the proof-of-work to prevent someone from forging bitcoins. Otherwise you could just say you mined a buttcoin and nobody would be able to say otherwise.
But yes, blockchains(and more generally merkle trees) shouldn't be currencies, or more accurately commodities(since they're currently way too unstable to be currencies). They should be used for reliable distributed event/resource tracking, of which currency is a small and relatively shitty subset.