Personally I appreciate why smart bitmixer's owner was, this person is great. He created the most amazing mixer and also he closed it himself. To make it more clear, only in 2015 year bitmixer was roughly mixing up to 60K-70K coins monthly (they launched service in 2014 year). Then in 2016, bitmixer owner stated himself that their website mixed 1 million coin. So what do you think, how many people were using it for just protecting their privacy during online shopping? I think maybe some percent only, max 4-5%. See how many websites were seized and how many still aren't and how many things are done without websites via different messengers.
Nowadays mixers aren't used for good purposes, it may was in 2014-2015 year just for the interest of mixer.
Currently mixers are been used by maximum community who have scammed someone and now want their bitcoins to get mixed up so that it might never be possible for anyone to catch them. Also most wider use of mixers are been done on the deep web or dark web where almost each of the site used a bitcoin mixer in order to mix their transactions. That is where the maximum illegal funds gets rolled into bitcoins and also spoil the reputation of bitcoin.
Mixers give boost to such illegal platforms and hence there are a lot of mixers which were been forcefully shutdown by the legal authorities. Bitcoins are naturally anonymous which makes tracking the transactions a bit difficult(but not impossible) and these kind of mixing services literally make tracking the transactions almost impossible.