I believe some people use mixers just because of being obsessed with privacy while at the same time preferring Bitcoin to, say, Monero. I'm sure that not all mixer users are doing illegal stuff. Some people might just want to ensure that nobody traces their money. Others might want to do things that are not illegal but are confidential or just something one would reasonably want to hide (financing a political party, spending money on something your close ones would not approve of or something like that). But there's a good chance that mixers are used primarily for illegal activities, I think, so I would not mind them becoming illegal.