Why don't you use CPanel, Plesk, or Ensim?

The reason we got into this business is we like administrating machines, and don't need or want graphical tools to hide the details. We do use usermin, and some custom scripts which let our customers manage their own accounts. 

Lots of other tools make it harder (or even impossible) to administer the specialized accounts that we provide.  When working on systems for other people, we often run into issues that would take merely seconds on our servers to change one line of a text file, but on their server, it takes the technician a long time while the customer is on the phone waiting for him to figure out how to do it correctly.