I am surprised that the issue of "doxing" as part of a social engineering attack hasn't been mentioned more.
Consider the scenario that it becomes evident (via psuedanonymous answers on stackexchange) that "Bob" is intimately familiar with a particular brand of routers/devolps using .NET/uses Firefox/provides other in-depth answers on a topic suggesting he's an employee at corporation x.
Answers on family.stackoverflow.com (under the same pseudonym) narrows his location in the world to a small section of a particular state, strengthening the likelihood of the assumptions above;
"Bob" also has a linked account (automatically merged... oops!) on hotsexysingles.stackoverflow.com which he is normally very careful about only accessing via a VPN (avoiding router logs!) on his home connection (otherwise browsing anonymously), but due to the auto-login now has a "last seen" of a few minutes ago (during work hours);
"Bob" had registered to several hundred so sites many years ago but never really interacted with many beyond passive lurking. Due to the auto-login feature combined with the "last seen" data, iterating through "Bob"'s other linked accounts (manually/automatically) it is now possible to determine that he has recently been looking at career, travel, and lgbt related so sites.
Given the presence of a 'last active' field, this may lead to manual/automatic enumeration of sites enumeration of "active" sites (vs "registered-but-never-actually-visit" sites).
While the scenario here became a bit more outlandish that I'd originally intended, the resulting harms that could arise here include blackmail, targeted phishing attacks, personal/corporate embarrassment, and assumptions being made about his upcoming career ambitions/personal life plans. As more sites begin to be included in the future (area51, careers, chat, etc..), this issue will be exacerbated.
Mitigation against this could previously have been achieved by explicitly choosing not to log in on that particular sub-site, thus avoiding triggering an update of "last active" data. Unfortunately, auto-login (thus auto-update of the 'last active' field) now leaks this snippet of data, through no explicit action of the user.