Scarcity alone is not enough, but being scarce + being desired by some people is enough to attribute value to any resource.
Yes they have value because they are desired. Tulip bulbs were valuable because they were desired, and desired because they were valuable. My problem is that if a thing is ONLY desired because other people want them, then no rational price exists. Tulip bulbs could be $100 or $1000 or $10,000 and none of these prices is any more rational than any other price.
Actually, there is a rational price, which is the commodity value. If it has no commodity value then it is zero.
The solution to this is to peg the value of the coins to something with stable value like gold. I think this should be possible without introducing any form of centralized authority.
The beauty of BitCoin is that no central authority can manipulate it. When enough people use it for transactions, it will stabilize. Right now it is in pure speculation mode.