I agree in fundamentally with the person’s development layout
a. content – live site – end users
b. New module or Style changes – Development system.
The problem is in the real world, in many cases, you will be porting over a non-DNN site to a DNN site, and these sites will contain content. I usually use that content for testing, To test without content, in my mind, is a incomplete test and a disaster waiting to happen.
In my case I have come to realize that when porting sites, the export template is probably the best bet, just realize you will have to double check the work. Once it gets to the live site, content can be changes added to by your users. Caution- if you are using html modules, and want to maintain font, heading, color consistency, you will need to add content and style it our. Then let your users know all they should change is the text and not touch the font styling. (I have already had problems with this)
Lastly, to your specific problem – It has appeared to me that in a parent child relations, the child portal does not allow someone to register with the same user profile of a user on the parent site. If this was the case , importing into a child with dnnadmi setup and the parent had a dnnadmin set up , you might experience the result that you described.