If you have a lot of users, e.g., if you are an ISP, you will save a lot of administration time by farming out tasks to users. They can connect to the Post. Office web site with Netscape and change their password, delivery mechanism, forwarding info, vacation messages, etc. Security with Post. Office is much better than with sendmail.
Office is obscure.
Not too many people run it. Even if someone does figure out how to attack Post. Office, the program doesn't run as root. It runs as a regular Unix user with ordinary privileges. Hence Harry Hacker's instructions won't be running with superuser privileges.