This is my understanding as well.
I understand the advantages of having children of a custom style - the template changes in the top custom style trickle down to the children styles (unless the child style had it's own custom template different from the parent).
The only difference I can see, is that when a custom style is under the default styles, is the path to images is set to default. But any style can have a image path set to the default or custom image directory, so I'm not sure that makes any difference at all.
The main reason I started this thread was because of you input.. I don't quite understand what you mean?
http://xenforo.com/community/threads/how-many-styles-do-you-have-installed.11478/page-2#post-153066