All battery cases have there benefits and I have even tried mophie. I'll list why I decided to go with Apple Smart Battery case.
1. You can use your lightning cable. Mophie uses a different type of cable.
2. Apples case is integrated into iOS.
3. It's all one piece. You don't have to worry about parts breaking or losing a piece.
4. It not bulky all around the case
5. Maybe one of the biggest reasons but Apple even integrated a passive antenna into the case so cellular reception doesn't suffer.
Just my personal thoughts.
I only can agree with the lightning cable being a good design choice. Everything else is very questionable or just very "apple".
point 2. the case being integrated in the OS, that's nice and all but i think that's to make up for the the fact that there is no way in knowing how much battery life the case has otherwise.
point 3. It's all one piece. Lol, whut? The likelihood of either of those two things happening is extremely low. the silicone of the apple case tearing is a higher probability than what your suggesting.
point 4. It's not bulky all around the case. And it's design suffers because of that choice.
point 5. If metal bumpers don't really affect cellular reception, then I'm not seeing battery cases doing that either.
For a $99 retail case, you not getting a case that fully charges your phone from dead and it's only offered for one iphone model. I don't think it's worth it, but I could say the same about most of apples official cases.