People already have the right incentives to not re-use coins (privacy).
This is not the case. My privacy is not an incentive for someone who wants to pay me coin, nor for someone whom I want to pay coin to. It is only an incentive for me, and I need the cooperation of these other parties to make it work. In the absence of some further incentive that works for all the parties involved, privacy simply has not happened and evidently will not happen.
It's nice to think it's "coming" in these newer clients, but my privacy is more secure, AFAICT, if when it does so people have some substantial reason to *USE* and *PREFER* these features of the newer clients. Otherwise I wind up out on the short end of the stick dealing with people who don't want the new client for some reason and who aren't sufficiently inconvenienced by my lack of privacy to care.
This. Privacy doesn't end at a single persons addresses. It involves everyone one you interact with and everyone they interact with. One can be perfectly anonymous and if the people they interact with are not well that can provide an indirect method of identifying transactions.