Reusable passwords have been in use for many years. Yet, this mechanism hasn't changed much and hasn't kept pace with changes in computing technologies. Cleartext password traveling over Local and Wide Area Networks and the Internet can be grabbed from networks using sniffers available from various BBSs and the Internet. Lax security procedures, lack of standards, and general apathy about the need for good security measures also contribute significantly to the weaknesses of reusable passwords. The weaknesses of reusable passwords are many.
Statistics show that many users tend to pick very weak passwords.
Experience shows that users share their passwords with colleagues for various reasons. Many password mechanisms do not provide for minimum password length, password reuse, and password expiry. When mechanism offer such facilities, they're often not implemented. There are some very basic things we can do to enhance the security of reusable passwords. Developing and implementing standards such as minimum password length, password history, password expiry and others, can improve upon what can otherwise be an ineffective security mechanism.