From recent news in India it is heard that bitcoins are used to sell drugs..I know its a currency and it is also used as a normal money... but these can create a bad space in people's mind that bitcoins are used to sell drugs..so they will not use bitcoins... Conclusion is that BTC is also a currency which is having a high value in the market..and bitcoins must not be used in these illegal activity which can reduce its usage and value.
It will be possible because of bitcoins' being decentralized. Anonymity is a criminals best friend so it would attract a lot of people on that kind of business. But if ever that happens, government would ban cryptocurrencies and it's value would eventually depreciate.
This is why bitcoin should not promote anonymity, it will soon be the cause of bitcoin will be a tool for criminal activity.