I'm surprised Apple have done this, I still feel battery cases are largely a niche, I certainly don't know anyone who owns one, and I have no use for one.
I had one for my 3GS. After a year, the 3GS started dying after ~10 hours of sleep. If I used it at all, it would die after 5-6 hours. Getting a battery case doubled its battery life for me, back to about as good as it was for the first year.
My 4S didn't have the same problem of the battery having serious problems after one year, so I never needed a battery case for it. 6+ has also been problem free.
My particular 3GS might have just had a defective battery, and it didn't become apparent until I had already owned it for a year.