A currency, virtual or not, only has value if people perceive it to have value. There are a lot of old currencies that no longer exist or have general value because of this lack of agreed value on the part of the receiver and the giver. You won't get far trying to pay for something in Italian Lira, East German Marks nor the Roman Denarius. Coinage of these defunct currencies has specialist value for collectors, but not a lot more with the exception of the value of the metals themselves.
A currency is nothing more than one-step away from bartering. Rather than me having to trade onions for socks, instead I simply exchange onions for an agreeable number of tokens, and later exchange a agreeable number of these tokens for. The important part is that all parties agree that these tokens have a value, in this instance it's the number of onions or socks they are worth. The next most important point is that while there is a level of trust implied, people will always game the system therefore the tokens must be hard to reproduce which is where scarcity comes in.
In some ways, bitcoins are closer to the origin of currency than the current money markets and banking system where "money" is moved around and between databases on the pretence that it actually exists despite the fact that "calling" all of the money will find an enormous, impossible to fulfill, deficit.