The mixing sites are used mainly by people earning their coins from illegal deals, so i think with time governments will illegalize such serv ices, even if it is legalized till now in many countries.
I don't think the one who are involved with darknet marketplaces only uses tumbling services, for example coinbase users can't gamble with their bitcoins so they might use this mixing services to add anonymity to their transactions and enjoy gambling without any restrictions. There are also many other reasons like someone love to remain anonymous completely when sending/receiving bitcoin but don't think the inherent privacy bitcoin have is enough to protect his personal details might also use this services.