I buy a bitcoin, I send it to BTC-e.com and sell it for $800. Now I have an $800 USD balance on my account. I wait for 4 days then buy Litecoin and send it to my offline wallet.
Now, how would you track that? Note in my HN profile I have a BTC & LTC address that I've done exactly that with more than once. I would love for someone to show me the blockchain transactions that prove that's what happened. Alos note, this is pretty much how the SheepMarketPlace thieves got away. Some guy tracing the bitcoins until it was revealed that he was just following a BTC-e.com's internal operations address. Also note that you don't need to give BTC-e.com anymore more info than a valid email-address. Just get your coins from some other place like mining or whatever, send them to the deposit-address given to you on BTC-e.com.
For people not familar with bitcoin, consider this real-world example. I steal a $100 bill from your wallet but you have a paper-thin tracking device inside it. So you watch it physically move around in the world. Now, if I go to an Airport and exchange the $100 for 89 EURO... you've lost track of the original thief. The thief still is holding the funds he gained from the theft and you're just following a $100 around in the Airport exchange system that may end up anywhere. You know where the $100 bill is but you'll never find the thief. Mix this with stuff like localbitcoins.com(equivalent of just finding a random person on the street that will exchange currency) and you'd have a pretty tough time finding out exactly how the original person spent the funds.