In a company it is dangerous because of the chance for unauthorized access. Every employee should have their own network password that is unique from anyone else's. Too many times someone uses the same password for most things or one password is used for everyone's login. Then, when an employee leaves the company they would have the information access to the computes. In the event of a breach there would be no way to know who had done the damage.
This is especially dangerous if the shared password has administrator rites. In that case the person could even change the password so that no one else had access.
One more note on passwords. Make sure you change them or remove the user accounts whenever you change employees. Yes this is a pain but it is better to be safe than lose important data.