Its just individual preference really but some of my quirky reasons are:
1. I don't know how old any one battery is because they're all mixed up so don't know when they might go caput because I haven't had one go out on me yet.
2. Same goes for the PCCs except I do know when I got those but don't know their life-span.
3. I like the ritual resembling when I carried a pack of smokes.
4. To ensure I won't be caught out without a charged battery... the dread of Type A vapers.... carrying multiple PCCs and batteries loads me down too much. In the case I carry them in they're way lighter.
5. Then there's the juice thing. I hate carrying a bottle of it or filling away from home. If I'm gonna carry multiple filled tanks and have enough batteries, I might as well carry those too saving assembly.
6. If one of my tanks does leak, they're all in a closed up case so nothing gets on me or my PCC.
7. Which brings me to last (I think)... leaks... Joye says not to put the battery in the PCC in reverse, which I had been doing, as it may cause leaks... so when thinking about where I could carry two plus an empty PCC... I just said heck with it... I'm going to carry as many charged ones as is comfortable.
I'm sure I had other reasons back when I decided to do it this way but I can't remember them now. Now you have some insight into Type As.
Edit: Oh, I'm obviously a heavy vaper.