This is one of the things that drives me crazy about vapocurrencies. "Mining" is exchanging actual work, in the form of energy burned, in return for nothing of intrinsic value. That same energy could be used for lots of more useful things, including processing SETI or Kepler data (which has a very small but still far larger chance of yielding something of actual value), rather than doing nothing more than contributing to global warming and the entropy of the universe. At least the tulip bubble looked pretty and was useful, if nothing else, for fertilizer.