It shows that the groups who go after and prosecute these people don't have the resources or motivation to get to all of them.

When you have a very significant target of the US (near the wanted level of Bin Landen) or China talking freely on Tor without getting caught, then perhaps you can rule it as being seriously anonymous.

I don't know about other countries, but in the US I generally only hear about people getting prosecuted for CP and similar crimes when they are caught in the act or when somehow the computer files get accidentally discovered or reported by some 3rd party.