Interesting thread for this newbie. I think I get that aphoto has been designed to be usable across multiple apps with performance and/or rich info stored. There's certainly value in this, and I'm not bothered about the size of the aphoto in itself - I would not keep my source/final versions in a proprietary format.
But myself am more interested in auto-sync photo editing between ios and Mac, i.e. an alternate to the LR/PS cloud offering. I think the aphoto format seems too large for cloud upload speed and storage...maybe it's not too bad if I have to save to a tiff earlier on, but anything that help streamline that workflow would go a long way. I think this is a market area that some vendors will try to jump into...Affinity is already there with a great app on the iPad, but I think the weaker point there is the workflow and ease of integration/use of (non-iCloud) cloud storage.