For example, today I can get 1BTC for ~$100. Say I held onto that 1BTC for a very long time and eventually bitcoins became the earth-standard currency. That 1BTC would now be worth significantly more than $100 (say, now I would be able to buy a house with it).
You might argue that this is how banks work (interest etc.), however banks earn you interest by lending your money out to other people who contribute to the GDP on your behalf.
It all comes down to buying power. http://en.wikipedia.org/wiki/Purchasing_power
Don't get me wrong, I think something like BTC is a great idea - our human nature just makes it impractical (not impossible). Things like money-hoarders and deflation need to be addressed, which at the moment are not.