I would remain suspicious if there was any identifying or unique information in cookies after logout. Ideally, logout should delete all cookies.
A major faux pas like leaving your uid in the clear in the cookie after logout certainly seems to bother us, but I don't think users (even savvy users) care about leaving some cookies behind. For the record, I've installed various opt-out browser extensions in the past (only to switch computers/browsers and forget to bring them along)--I don't think my views are pro-cookie or even moderate.
In most contexts, that is true. A Slashdot cookie is just a line in a text file until you visit Slashdot. But a Facebook cookie is sent home every time you visit a page with any FB spam on it.
The mysql.com malware is trivial. Hitting Facebook would get most everyone, users and not.