Q: Why not make it completely integrated?
iJoomla Ad Agency is not a CB plugin. This is an independient extension and we are working in order to make it compatible with CB and other cool extensions.
Q: Why force users to have a separate email address if they are already registered in CB? Could this be so difficult?
This is a Joomla! limitation. On Joomla! the same user couldn´t be on two groups at the same time. So, if you are a registered user (registered group) you can´t be an editor, admin or advertiser at the same time.
Q: I can envision a site owner perhaps wanting users to register separately, but why force it?
Well, there are a lot of sites where you need to registrate on two or more areas/sections. This is not the best but it´s necessary for technical reasons: see the next answer.
Q: And if you are going to automatically register Ad Agency users in CB, why not allow existing CB users to "add" an advertising account to their profile, or at least allow site owners to choose that option?
We are still working to give you a solution for this. We are waiting for the new stable CB version in order to improve this.