A good password has the problem of being difficult to remember. And sometimes you might need to get in to a system where the root password is long forgotten (or left with the system administrator before you).
Luckily there are ways of getting access to systems without having the password. This is of course in a sense also a security risk. That's why you should always be aware that having unattended physical access to a computer system means the same as having root access to the operating system. Unless the information on a system is encrypted, it's only as save as the room it's in.