And if it turned out to give you 20BTC but the USD/BTC rate sank to $5? Clearly some people here will call that a good deal, but others would've preferred the cash.
If you are buying an ASIC miner, you have already decided to buy BTC. The question then becomes, what is the best method to acquire BTC?
For every point on the USD/BTC exchange rate, 13 BTC is worth more than 10 BTC and 20 BTC is worth more than 13 BTC. If the device gives you 20, then you would be better off than if you bought a device that gave you 10. You would also be better off than if you had bought the 13 BTC from MtGox and held it.
Now if the exchange rate drops to $5, the problem was that you decided to invest in BTC, even though you may have picked the best way to do so.