It depends on how it is used. In the US currency gains of less than $200 on transactions involving the purchase of goods or service by an individual (sorry for profit businesses this doesn't apply) are exempt from any taxes.
This is why if you go on vacation to Mexico for example and exchange $100 for pesos, and before you buy a beer the next day, it doesn't matter if the exchange rate rises slightly. If the pesos you now spend are worth $101.20 you don't have to keep a record of each transaction, amount, time, corresponding exchange rate, and then load all that into your tax software to compute a currency gain of $1.20. For exempt transactions, it is considered a de-minimis gain and you are not required to report it or pay taxes on it.
As an example lets look at a consumer who buys 4 BTC from coinbase at $500 ea and later spending those to buy a $1,500 computer from tigerdirect for 2.9 BTC ($517 exchange rate). Today under IRS guidance there is a $49.30 gain and it would need to be reported as a capital gain tax. The problem isn't so much the tax but all the complex of the recording and computing of that tax. The time, exchange rate, and amount of each purchase and spend needs to be recorded and the capital gain schedule filed out, the gains have to be categorized as long vs short and the applicable rates applied. If it was treated as a currency then the transaction would simple be exempt. Not only does the taxpayer pay nothing they don't need to file anything.
Of course right now Bitcoin is appreciating (or at least has the potential to appreciate) and the gains may be more than $200 so it may not make much difference, however in a decade or two if/when Bitcoin is much larger and the growth rate much slower it would be much simpler for most average user of the currency if it was treated as a currency.
Regardless of it one wants it to be treated as property, there is no logical reason for it:
* You exchange $100 for pesos and spend them. Treated as a currency (and in most cases is exempt).
* You exchange $100 for euros and spend them. Treated as a currency (and in most cases is exempt).
* You exchange $100 for pounds and spend them. Treated as a currency (and in most cases is exempt).
* You exchange $100 for yuan and spend them. Treated as a currency (and in most cases is exempt).
* You exchange $100 for yean and spend them. Treated as a currency (and in most cases is exempt).
* You exchange $100 for Bitcoin and spend them. Magically it is "property" and capital gains need to be computed. WTF?
Think about it for a second. Have you ever traveled overseas on vacation or business? If so did you exchange USD for foreign currency and use it to pay for goods or services? Did you then track the exchange rate at the time of the exchange and at the time of each purchase, record all that in a log, and compute the total gain if any ad at the end of the year take that log plus the logs for all other trips and add it to your tax return? I am guessing not.