Hmmm.

I'd restate the highlighted portion as:

What's the benefits of participating in mixing someone else's coins, when no one else has to mix yours?

Even as someone who despises the concept of "tainted coins", and totally embraces anonymity, I have to admit, this proposal bothers me a little. Particularly the "opt-in" part.

I have the concern that by clicking to opt-in to mixing, I might wind up with the large majority of people I mix with being those trying to hide the source of stolen funds. What benefit is that to me? Sure, my financial history is obscured, but if that's achieved by hiding my financial movements among those of people with legitimately criminal financial activity, have I really gained anything?

If the rebuttal is, "well, most people will choose to opt-in" (which is questionable), then what's the point of the opting-in? Why not just make it a built-in, mandatory part of clients and be done with it?

I'm convinced passive coin mixing won't work unless it's automatic and widespread. Which means I'd be more supportive of an opt-out (or better yet, mandatory) client-level mixing methodology, but I still think the best way to do automatic, widespread mixing is to alter the protocol.

As a bonus, this would force those people who insist on having traceable transactions to go make their own fork.