Single Sign-On - The problem

In the past, multi-user computers have always stored all user accounts locally, even when these computers were part of a network.

fig. 1 Network with a central user database
network without a network information service

The user "Tom Jones" has a separate account for each computer.  This means that...
Some argue that single sign-on is unsafe because the user only has one username and password.  But users are more likely to write down passwords, if they have a lot of them or if they have multiple username/password combinations on the network, and less likely to write down this information if it consolidated.  This goes double if network administrators are also enforcing "strong password" policies.  

Another angle on this problem is amount of operating systems and services on the computer network.

Fig2 Various services/OSes using a centralized account server
Different services using LDAP

In this scenario, users are able to log into web-based applications on the webserver running Apache webserver using their LDAP accounts/passwords, but those same users are not allowed to log into the server using ssh, that is restricted to administrator accounts only.  This is archived by configuring Apache to use LDAP as authentication module.  This ability is buildt into Apache 2 and comes as an extra module for apache 1.  The mail server uses Sendmail SMTP and Cyrus IMAP/POP servers for user account varification also, and that to points to the LDAP server for accounts.  The third machine, the Login ( or shell ) server allows any user to login directly to the server.  It is configured to simply point to the LDAP server for system accounts.

Without some sought of centralized server, every service would have a separate password.  That is, the user would have a "email password" and a "web password", etc.  Redunancy on the service level would be next to impossible as well.