That depends upon what you mean by the phrase "US economy". Does this economy belong to the government or the taxpayers? I say the latter. Giving the consumer options can never do anything except benefit the economy, even if that means giving them the option of taking part in a competing economy that lays over top of the first one. This may even IMPROVE the traditional economy in the long run because giving the entire marketplace some competition will bring some accountability -- the traditional marketplace will have to correct itself in order to survive.
Don't fool yourself. A government is a glorified shopping mall management company -- a business, just like any other -- and you can not honestly tell me that the US Government is performing the job it has been paid to do. Individual voters are left choosing between one of a whopping two options, tailored to the interests of those willing to pay a little extra. The marketplace we were born into has ceased to serve our needs and somebody has come along and offered us an alternative... if US Government-owned marketplace wants to regain our attention the should be forced to earn it. Maybe BitCoin will give the system the shock it needs to realize what it has become.