Not why do you use them; why do you trust them? I remember reading a post here along the lines of if your OPSEC is tight, it really shouldn't matter if a market you use is a honeypot. I can see the sense in that. It seems weird though, that so many people are happy to trust unknown third parties with something they consider potentially incriminating. Am I missing something?
The same reason why they trust anonymous drug dealers on the internet.