I think that @o_e_l_e_o and @20kevin20 said correctly why mixers should exist legally and they changed my mind. In what I really agree is that banning them, doesn't mean that it will damage the criminals. They will find another way like monero etc. So yes, since privacy matters, then mixers matter too.

P.S, great debate for your signature campaign, o_e_l_e_o 

Yes, I agree, but as a conclusion of the posts above you, you can use a knife to make a great dinner or to kill someone. It doesn't make the knife illegal, but the person who uses it to murder.