I use private add-ons, because it makes your server unique. For example, if you want to go to a City RP, then you wouldn't go to the makers, but to someone elses. This can be a good thing, but for the creator you will be missing out on people. They're also useful if someone steals a build, because they cant use it without the private add-on.
I also have some that are abusable. If it got around to some servers, the admins may not be able to keep control. However I trust that I will be able to.