I should probably preface this by noting that I don't carry batteries on my person. If I did, it would only be one extra cr123a, probably in one of those storage capsules.
In any event, I do carry spare batteries in my bag. Rather than letting them bounce around, I carry them in a battery case. It's convenient, keeps batteries from rattling around, and offers easy access to the batteries themselves. I purchased orange for high-visibility.
Keeping spare batteries in my bag is one of my few habits that fall closer to the "two is one, one is none" philosophy. Batteries are often finicky and fail far easier than other gear. As such, having a couple spares around increases the longevity and usefulness of my lights. While I'm going to have a hard time catastrophically damaging one of my knives, a flashlight can quickly become useless absent replacement batteries.
I should also note that I've tried to buy and carry lights that use the same type of batteries. This means I can carry spares for all my lights without being forced to inventory a number of different battery types, saving time, space, and frustration.
For the few dollars I've spent these little cases have made a world of difference.