That would work against individuals who are snooping what their former coins are spent on, but would it do much against a government that uses some fairly serious resources to analyse the network? For them, it's just extra information to statistically link different addresses. If an address sends money to the laundry, then some of the payout addresses of the launderer in the near future must belong to the same owner. With enough such events, they will eventually figure out which addresses belong together.
But let's assume that there are indeed third parties that are secure enough that you can route your transactions through them, reliable enough that they won't steal or snoop, large enough that its incoming and outgoing transactions cannot be matched by outsiders, and sheltered enough that governments cannot demand to see their records.
If so, what's the point of using bitcoins? Why not simply send money to this third party, and tell them to pass the money on to your heroin supplier?