You're right about users. There's no need for other users. You can control all of your domains from one user name - and there's nothing wrong with that.
However if you wanted to be able to give out FTP/shell access to one of your domains, but not all of them - it would be easy this way to give access to just that domain.
For example, I host a domain for a friend. I set up the domain under a seperate user name, so they have access to their files, but not mine. I also set up a web ID so they have access to the Panel for their domain (I'll explain about that more in a bit..)
I'm not sure if there really is a security issue with having all of your domains under one user...or if it's better to speared it out over several. Becuase all of the users are apart of the 'group' they have some shared access.
As far as giving access to an other for one domain, you'll want to set up a Web ID. in the Panel > Billing > Account privelages. You can allow a web id (existing or a new one) access to the parts of your panel you want. You can grant access to just E-mail for the one domain, or mysql, E-mail, etc. It's pretty customizable.
Hope this helps.