System users
Placed in the wrong hands or manipulated by scripts, user accounts can be used to wreak havoc. And remember, while we may tend to think of users as being human, they need not be. As well as root and the web server user, for example, many system users are created by new packages with privileges to run processes.
Human or not, our responsibility is to contain user accounts so they cannot create direct damage, nor be hijacked. The level of control we have over this will depend on our hosting option, but an awareness overall is a thoroughly sensible thing.
Shared human accounts
Follow the logic. Shared user accounts lead to an increased risk of passwords, else maybe an authentication key, falling into the wrong hands. Not only that, they handicap troubleshooting because they obscure logging.
What is more, human or not, likely you and other users should have entirely different privileges. Never give any user, including you, any more privileges than is necessary because to do so jeopardizes...
