The danger of regulating cryptocurrency to the point where only certain types are allowed (this is where it is going) is that it is a restriction on freedom of expression. A very famous case made its way through re DeCSS where the government was not allowed to ban the software. I think it's a pretty straightforward precedent that the USG at least will not be able to prevent the creation of cryptocurrencies like Monero, which have true anonymity as a goal. The future is exciting. The government is our bitch, don't forget that.