If you convert your USD to a foreign currency, then gain or lose money due to fluctuations in that investment, then convert it back to USD, you pay taxes on gains or deduct losses from income otherwise taxed. BTC is no different from stocks, forex, or any other non-US dollar investment, it varies in value over time and those variations are taxed as capital gains (short or long term) when you cash out the investment.
If you've got vendors willing to accept payment in BTC, or foreign currency then you effectively avoid the capital gains issue, unless you get large scale about it and then you may have to pay tax on a barter transaction (e.g. say I purchased, mined, or otherwise acquired 200 BTC back in 2009 and in 2016 I bought a house with them, there will be tax, whether dollars were involved or not, it's a barter transaction and will be taxed at fair market value of the goods exchanged.)
OP is bitching that nobody accepts BTC, boo hoo, convert your BTC to dollars and they will accept it all day long, but the dollars come with a capital gains tax. If you have markets that _do_ accept BTC, then kudos, enjoy your shadow economy while it lasts - if it gets big enough, it too will be regulated and taxed.