BTC though: Unstable rates...
USD has unstable rates. Yesterday 1 $ was 1.15 mBTC, today it is 1.05 mBTC. Can't use a volatile currency like that.
(it's all a matter of perspective)
USD has unstable rates
Compared to what, the rate of 1btc

National currency usually doesn't fluctuate in value +/- 10-12% in a day

That would be horrific