There are a few good reasons I can think of for writing your own script for generating accounts on a Unix box.

• In one environment where I did create a custom script for doing this I had a requirement that clericals were going to be adding in new accounts.

  To accomplish this I created a CGI that asked a few questions and used business rules to create the account.

• As in above if there are policies that you want to enforce that an adduser command has not been set up to deal with. For instance, spreading user accounts across multiple servers, putting users home directories across multiple mount points based on initials, etc. etc.
• In one case I administred a system where the adduser command was capricious, cantankerous, and downright unreliable.