Using bitcoins for anything illegal is a bad idea.
This is true only today because you can't pay the rent, or buy much of anything, with bitcoins.
However, if we imagine a world where bitcoins can buy anything, then the classic cypherpunk "perfect crime" scenario begins to sound plausible:
1. Kidnap Rick Falkvinge's daughter. (* Note: I have no idea if RF has a daughter; I choose him as the victim solely because of his babble about how there is no such thing as 'illegal trade'.)
2. Demand payment to a bitcoin address. Or Else.
3. Non reversible payment made, you publish the coordinates of the girl.
Curiously, when people propose mechanisms to prevent step (4) from happening -- e.g., bitcoin blacklists -- people get upset.