If users follow best practices and never reuse addresses and use clients with good output selection algorithms they don't need a external mixing service. The only time when mixing is necessary is when they want to create an output larger than any available inputs.
I don't agree here - after all, at some point in time you simply won't have enough large outputs anymore because all outputs keep shrinking. Thus you have to combine smaller outputs from time to time in transactions (except if you want to "throw away" too small outputs regularly), which then helps to associate all those to you.
Furthermore, another advantage I see in a mixing service is this: Somewhere I have to get my initial coins; and if I don't mine, then buying is what I have to do. If I buy for instance at Mt. Gox (yes, I know about localbitcoins and the like, but what if there are no sellers for cash in my area?), then at least *they* know theoretically my real identity and could connect me to everything I buy with the coins. Using a mixer can break this identification from my real identity to the bought coins.
I would be very delighted to see a p2p mixer based on this or a similar concept developed. After all, it seems not too hard (compared, e.g., to the p2p exchange everyone is talking about at the moment), and would be very valuable in my eyes.