Sure but, if you can mix them in with coins that have passed through other hands, is that not just as good?
Possibly better, since backwards attacks are possible. Consider this, you buy drugs with btc from a mined address.
Now the rest of your coins, mined from that block, are all tainted with your drug transaction, and can still give you away back to that transactions when you otherwise sell them.
Maybe you worry less about this? Maybe not? In any case:
https://en.bitcoin.it/wiki/AnonymityIs an excellent link
Suppose the overall majority of BTC trading ends up being drugs (a plausible possibility if word of BTC spreads like wildfire across drug users)... there won't be enough "clean" coins to really dilute the history.
In fact, there'd be a disadvantage for large sellers: if BTC were overwhelmed with drug-related popularity, a "fed" could merely spend 5 BTC on a little pot, and then watch where that 5 BTC went, and as those 5 BTC got combined and mixed around, they could arguably claim that every other BTC that the original 5 touched or mixed with soon thereafter is also drug money. So if 45000 BTC hit a local exchanger that included that 5 BTC, they'd be making life miserable for the exchanger.