Instead of having a case like this, I would settle for an iPhone than was a little thicker. I see diminishing return on going thinner. Just put some more battery in the phone.
Selling the case increases Apple's revenue and makes their phones sexier in the store. Apple's strategy is based on the idea that a lot of customers will buy this after a period where they were unhappy with battery life and did not purchase a non-Apple branded battery case.
So, it's a band-aid, just like those hubs for the Mac Pro.