Probably not many. But, the point is that they can't use violence to corner the market for distribution anymore, with something like Silk Road around. Provided

Probably not many. But, the point is that they can't use violence to corner the market for distribution anymore, with something like Silk Road around. Provided dealers remain anonymous, there is no "turf". This simultaneously cuts down on drug-related violence, and takes money out of the pocket of cartels by making the drug market more competitive. It's not perfect, because if they still control supply they can still exert pressure on distribution. And, while Silk Road may be anonymous, obviously buying drugs in bulk from the cartels is not. But it's a step in the right direction, however small.