... 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.
I don't think you realize just how non-anonymous the current bitcoin client really is. Most people would prefer that the amount of coins under their control not be so easily discerned from the block chain. Do you walk around wearing a shirt that announces your net worth to everyone? It has nothing to do with people engaged in what some may consider bad behavior.