First of all, it's fiat currency, not flat currency.
Second, no matter what fixed value you choose to link your currency to, be it gold, oil, or bacon, you still get the same problems that you get when you back your money with gold.
I refer you to this point:
A gold standard leads to deflation whenever an economy using the gold standard grows faster than the gold supply. When an economy grows faster than its money supply, the same money must be used to execute a larger volume of transactions. The only ways of achieving this are for the money to circulate faster or to lower the cost of the transactions. If deflation drives costs down, the real value of each unit of money goes up. This increases the value of cash, and decreases the monetary value of real assets, since the same asset can be purchased with less money. This in turn tends to increase the ratio of debts to assets . For example, assuming interest rates remain unchanged, the monthly cost of a fixed-rate home mortgage stays the same, but the value of the house goes down, and the value of the money required to pay the mortgage goes up. Thus deflation rewards cash savings.
Let me make this clear. This makes property less desirable than cash, which means that people will start hoarding cash (because unlike property, cash will remain constant in value), which removes money from circulation, which decreases the money supply, which makes money more valuable etc etc. Investing money becomes less desirable, in other words, which freezes up the economy.
In addition, people who go into debt are screwed, as the debts increase in value. Do you
really want to be saddled with even more debts that you can't repay in a few decades?
Third, as said above, a backed currency's value is dependant on the value of the material used to do the backing. If that value fluctuates, (and as the chart for the value of gold posted by BW shows, it can fluctuate wildly), your currency's value fluctuates. Tell me how that is a good thing.