If we talk about the general use of one computer, I would suggest partitioning the disk and installing different systems for different users.
Partitioning different OS in one system isn't possible for every computer user; lay man. I've read about it in my school days and it involves processes that should be studied before installing different system in the computer or an expert can handle that as well for the user. Though, it's a valid point, but not everyone can easily achieve that security measure.
Regarding Op's post I'd say that sharing our password sometimes to people we trust doesn't seem wrong. I could remember a morning I woke up and forgot my PC password, I don't know what happened to my memory, I tried multiple times to no avail. Luckily my brother has access to the laptop, I had to inform him about my login troubles and he recalled vividly the right password to access my PC. So, aside the disadvantages, they are benefits involved in sharing sensitive password to others. Even the private key, can be shared with a good spouse for future purposes. Nobody knows what can be occur.