For some reason, people think that having an interchangeable battery is only about being able to have a second back up battery to extend power. Batteries have a limited life cycle. They die. I've run through several batteries on my current S3. If you are a heavy user, your battery run time will eventually get less and less. I use my phone for offline navigation, on trails. I keep the different TOPOs on different sd cards. These phones are spendy, so I prefer to keep mine till the wheels fall off. My S3 is running 5.1.1 and I have a Moto Xoom running CM 11 (4.4.4).
I don't see any problem with batteries getting weaker! It's a non issue with removable batteries.
I just go on Ebay and order fresh high quality new batteries!
No problem what-so-ever.