This has been brought up many times before,
- Search: Can't Log Out
- I can't get my account not to auto-login at work
- Why can't I log out of Stack Overflow
- Undesired auto-login to meta?
- Improve SE OpenID logout security and user experience on shared or public machines
and I think it is not getting enough attention.
Hundreds, if not thousands of accounts are completely available to passersby, unbeknownst to the account owners because the details of auto-login are stored in local storage on devices they don't own. My account alone is probably still saved on three+ multi-user/public computers I've used before I realized my account details were saved on the device.
We have one Mac for development at work, and when someone wants to use StackExchange on that Mac (without logging into my account unannounced), they have to open Firebug, logout, delete all local data, refresh, delete all local data, and then log in to their account. How many people have failed to notice this and had their accounts basically hardwired to a device?
It sure is helpful if you have one account that is only ever used on one device, but I can't see any sense in having this feature considering a lot of us use many different devices. Smoothie King customers and the patrons of the public library do not need full access to my Stack Overflow account and information. Thankfully this site does not store personal or financial data.
A screen recording of my concerns: http://youtu.be/QqkMWIVjMWg