US dollars are massively manipulated and counterfeited and inflated. Digital currencies can be manipulated but not on the level the government can. They cannot be counterfeited or inflated. 

Finally, as for storage, I am at a loss why people would trust a third-party access to their private keys to brokers like Mt. Gox. If you don't control your private keys, you don't really own the coins. Things are much more stable now since Mt. Gox. Again, new technology, new learning curves. Bitfinex has refunded all of the losses to its clients and Mt. Gox is looking at methods of repaying back a well. Like anything else, risk=reward. You can leave your money in the bank and have it lose its value due to inflation but at least it is "safe". That is until the government decides to freeze your account or repeats what happened in Cyprus or Greece.