I still have, and sometimes use, about 8-10 of the 510 slim JoyeTech and other brand batteries that are 280 and 320mAh. I use them at times for dripping while at home. Also, with my stash of Vision Nano clearomizers when I'm out places I know there probably won't be anyone else vaping. I'll probably always use them in some way, even though I also use 1100mAh eGos with Innokin 16D tanks and my iTaste MVP, too.
My female friends and relatives that I'm currently helping to switch to vaping, and those who already completely switched...preferred the small, slim size of the JoyeTech's to start. In fact, 5 of the 7 say they probably wouldn't have had success if they'd had to start with even an eGo battery and a little larger clearomizer than the Vision Nano. It's because of what they've said, and the increasing difficulty finding the 280-320mAh ones still available, that I've bought another 10 of them for future friends who may prefer them.
So, I agree, there will always be those who prefer the slim batteries, even if they're less powerful and need to be recharged more often. I figure...whatever works for them is great. The eventual outcome is the important thing.