Well, let’s make this as simple as possible.

A man sends his coins to exchange E and receives different coins C after a while. In your opinion, this is a bad practice.

On the other hand, if a man sends his coins to a dedicated mixer M, and the mixer sends those coins to the same exchange E, getting the same coins C back and then sending them back to the man, you consider it good practice.

So, what has that exchange E done aside generating profits for itself?