This area is really for architectural direction questions - sounds like this is identified as a bug with a fix scheduled for 2.0.7 (so not released yet), as per the GitHub issue https://github.com/magento/magento2/issues/2121 - so I am going to mark as "Accepted" if that is okay, to help me remember which issues I need to follow up further on. (Yeah, yeah, I know, I am behind!)
