For the individual:
Simple question, given the choice, ask yourself would you rather have a bank account that is totally private with no possibility of anyone gaining information about your financial transactions or another bank account that is maybe private, but has no guarantees against who your financial transaction behaviour maybe data-mined by?
Then shouldn't it just be an automatic, mandatory thing, hardwired into clients, forcing those who want to avoid mixing to find clients that didn't mix?
I think you have it all backward, even with today's with unsophisticated coin mixing services it is still near impossible to track the origins and ends of 99% transaction.
What make you think 100% of people would want to help anonymize wrong doing using bitcoin.
Why don't you take it one step further and wonder why not everyone is willing to run a TOR exit node by default ? . And why it wouldn't necessarily be a good thing ?
As if Bitcoin isn't revolutionary enough, it's required to provide 100% anonymous transaction while being ~95% anonymous already and criminals can easily achieve 99.99% anonymity with a small fee and some know how.
It's not required.
But if you're going to try to do mixing at all (which is what the OP was about), I don't see the point in trying to go for half-measures. Keeping such a system "opt-in" discourages the majority from ever using it, and leaves the honest people using it more vulnerable.
The reason not many people want to run Tor exit nodes is because no one really has to, and because of that, claiming your computer only connected to Super-Evil-Terrorist-net because it was running a Tor node at the time probably won't protect you. If all internet activity was automatically onion-routed by default, no one could be condemned simply on the packets their computer sent. Everyone would be protected. Those who wanted to broadcast the purity of their packets could, with enough work, arrange things so their transmissions were uniquely identifiable.
Don't get me wrong, I'm happy with Bitcoin the way it is. I just don't think "opt-in" mixing will ever have enough honest users to be worthwhile.
Edit: And again, I find the claim that "most people will use it," while simultaneously declaring it should be an "opt-in" feature, to be a little inconsistent.