My understanding here is that so long as multiple inputs are merged together in a payment, sooner or later, your full wallet history will be public (assuming there's no CoinJoin fog on the network). Perhaps it might be nice to write a part about privacy in the (future) payment requests subsection and link to it from here. Payment requests allow to send separate transactions as part of a single payment, providing a real solution here. Mike Hearn published a post about this previously: [Merge avoidance](https://medium.com/p/7f95a386692f)