I'm unsure if this anecdote will be helpful to you, but it's the best I can offer. During Christmastime, my large, extended family has a big get together with a fun gift game. All the presents get piled in the center of the room. Some of the presents are silly joke gifts, but others are quite nice. Everybody gets a number. When you get your turn, you can either pick a wrapped present from the pile, or steal a present from someone who's already opened one. Each present can only trade hands three times. All sorts of deals and conspiracies follow, but it's a lot of fun.

This year, one of the kids, a boy about 13, got a pair of battery cases for two different iPhone models. Though he had an iPhone, he REALLY wanted to get rid of the battery case so he could pick another gift. I played the part of the "too kind grown up" and "stole" the gift he was vehemently flogging. I don't own an iPhone but I figured I might be able to use them for parts for some electronics project (you can never have enough spare batteries and widgets ;). The battery cases were actually good, band name (Duracell I think) accessories, but absolutely nobody wanted them.

As much as people complain about phone battery life, if you can't get anyone in a group of adults and especially kids to want a free battery case, there may be some issue with the product.

Though battery cases make sense to you, and me, I've never been a heavy mobile user, so I've never had the need for one.