I really gotta wonder what sort of protection this actually provides. A tumbler only helps if a lot of people with legitimate BitCoins frequently mix them in,

I really gotta wonder what sort of protection this actually provides. A tumbler only helps if a lot of people with legitimate BitCoins frequently mix them in, so that people with coins from a mixer have a plausible claim of innocence. If mixers are primarily used by criminals, you've only anonymized which crime you've committed, you haven't really laundered the money. If those coins touch your real-world identity, you may not be incriminated in a particular crime, but you've made yourself into a suspect in a relatively small pool of crimes.