Bitcoin can be used as a main currency only a Deep Web. In every other place its status is the same with all the other kind of currencies. It is not only bitcoin which is used by criminals. Even, they mainly use other money and not bitcoin because the use of the other money is much more spread than bitcoin. That which can be assured using bitcoin is much mess than that which can be fulfilled with the other money. And this proportion condition even the use of it compared to the other currencies. A criminal don't ask for bitcoin. He ask for that kind of money which help him more in the achievement of his aim. And no one can prove that this currency is bitcoin. Cannot be proved such thing. There are hundred and hundred criminals that make crimes and are free. Who knows what kind of money is preferred by them? They are free and cannot be known what they think until will be captured by the law. To not tell even that it is very hard to be proved that all those in jail have told the true about this question (if asked).