Hi samcy.
A child theme allows you to add, remove, and override certain pages, css styles and even functions. The "core" functionality remains in the parent theme. For example, 3.1.7 had an issue where Membership subscription reminder emails weren't being sent (3.1.8 fixes that). Upgrading the parent wont affect your style or custom pages set within the child, but the email will start to work. Basically, the purpose of the child is not to replace the parent, but rather to augment it. I have used the same child through three (as of 3.1.8 just today) updates to ClassiPress without needing to modify the child in any way.
If you haven't already, you may want to have a look at this ClassiPress specific tutorial:
part 1 and
part 2.
I think it's safe to say that AppThemes recognizes that the documentation on how to leverage
action hooks (oh boy, that's a whole new can of worms) and child themes needs some serious work, and I know there are a lot of folks (I used to be one of them) doing the so called "stylesheetpath fix" and even creating new themes based on a particular version of ClassiPress (fine if you never want new features or fixes). I am working on some tutorials that will include real world examples based on support requests. My intent is to explain the "why" as well as the "how" without talking down to, nor over the head of, our average "enthusiast" customer. Sorry, no ETA at this point, as I just started formulating my action plan last weekend.