If I have two accounts, one with 0.5 btc (newly created, never tracked by authorities, purchased with a relatively disposable account from somewhere like virwox) and one with 1.5 btc (potentially tracked, need to launder it), I'd pay both into a tumbler and then extract into 3-5 different accounts (like 0.2, 0.45, 0.9, 0.18, and 0.27).
How do you prove, as a tracker, that those 5 accounts paid out to, involve btc from the original 1.5 tracked account? Especially when you consider the hundreds of other accounts being paid out to at the same time.
Still paranoid? Rinse and repeat.