Of course it can't. Though for person to person transactions and exchange, it's applicable. Both parties should be aware of dealing with a volatile value exchange and if both are i agreement then it shouldn't be a problem.
I agree that a currency with high volatility can`t be used as money. But in the case of the crypto currency, which has many other advantages, and which people want to use as money, this problem can be solved. One possible solution to the problem of volatility is to tie the crypto currency to gold at a fixed rate. This will require the consensus of the crypto community and the sponsor who is ready to make the physical gold market at an agreed price in the crypto. This kind of crypto, backed by gold, may even become a global reserve currency, especially if it was supported by the countries who are looking for ways out of the current hegemony system of the USA dollar.