Now lets say the same user wants to add 10 domains.
Actually, I do know what you are saying and have come across quite a few clients that have used add-on domains, to host multiple customers/websites under their own account.
Bad idea. I've seen this in both cPanel and Plesk and I strongly discourage its' use, regardless of the directory structure used. Any clearer?
As said, a Reseller type of control panel account is a
much better prospect - in fact, that's how I started in value-add hosting.
However, if you're not interested in a (more) secure environment, don't let me dissuade you.
Parked domains are, of course, something different.