They should configure their browsers to forget cookies within 30 minutes of non-use. Chrome can do this with the Vanilla browser extension, and I use it myself, with whitelisting for a few sites, to ensure most unwanted, long-living cookies get wiped clean regularly.
If I forget to log out, my account is open to everyone. If I forget to click "remember me", I have to sign in twice. Making systems that fail safely in case of human error is a good thing.
Although one of my favorite ideas was a system I saw at a hardware store. You could use their terminals to look up products. The terminals had a pressure pad in front of them, and as soon as you stepped off the pad, it ended the session, cleared the cookies, and logged you out.
Lee Valley. They're a Canadian chain that mostly sell high quality hand tools, cabinet hardware, and gardening equipment, and apparently they're pretty popular for woodworkers in the USA as well.