Mixing code is needed when you actively use mixer, so to make sure that you don't receive your own bitcoins from previous mixings.
I should've been clearer
[my bad]... I knew the usage of mixing codes, but I meant to ask what happens when your pre-mixer reserve finishes? Sooner or later you're going to rely on the funds gathered through the mixing service itself... Am I right?
- I'm trying to point out how that could directly/indirectly link your users together.