[bodyfour]

Two examples from my life:

For machines that I manage that are hosted by 3rd party VM providers I always use a separate root password. If the provider is compromised an attacker could modify the disk image I'm running from (say, with a modified /bin/su) and I don't want that to spread to other machines.

In the distant past when I was doing consulting I used to have passwords for multiple different clients. Obviously they need to be kept separate.