Well they are a battery add on business first. They can certainly see the growth potential to a storage only option but wanted to release a traditional battery product for their customers first.
From that standpoint it makes perfect sense and undoubtedly they will offer one. It is just weird how many "bad at math" comments there are in this thread as people ignore the value of the battery.
If you don't want the battery don't buy it. Offering a case that extends storage on an iPhone is a fairly big thing and will likely change things forever. So be patient and wait.
Never said Mophie had to be the one to make the thing, just saying that I'm surprised it's taken this long to add storage into a case whatsoever.
I have a portable HDD for iPhone that's made by Kingston, which when I got that, wondered why they didn't just create a case form of it.. It's thin enough, seems as though I could just strap it to the back of my iPhone, so it's strange that a storage case never came out until now.
I think the price is fair for what it is, technically, but it's still a lot for what it is as well- but thenagain, I think the starting point of cases is a bit over the top. ~50 for a plastic cover for a phone seems to be a bit much, and then add all these features from there.