... by default ?
What's the benefits of participating in mixing someone else coins ?
Say 98% of users do not have anything to hide and would prefer all transactions be traceable for the benefits of discouraging bad behaviors ?
I would much prefer someone who stole BTCs to pay a fee to shameless mixer than to help him unknowingly.
Bad behaviors like what? Donating to WikiLeaks and other politically incorrect causes?
Do we want Bitcoin to be a system that tracks taint of coins, or don't most of us share the consensus that the system as a whole would be better off without the notion of tainted coins, even if that means a few thieves will have an easier time getting away with their crimes? For everything you're asking for, there's MasterCard.