Maybe it's just the luck of the draw but I've never had any issues with batteries but then again I'm light on my mobile devices, using them only for calls and messaging 99% of the time. I can get 3 days use from a full charge on my S4 and sometimes up to 5 days on my A5... and I never switch them off or enable the built in power saving options. I notice the batteries do drain faster when mobile data is enabled but I seldom use that because I usually have WiFi all around me.
Ya, in comparison, I'm an extremely heavy user. For me to make it to bedtime and still have a running phone is a big thing without swapping out the battery. I also usually purchase extended batteries, etc. I do not mind the size. We even make private mobile apps for devices where I work, to do more work.
With my older HTC Evo 3D, I had an extended battery, and it could go about 2 days on a charge, barely, if I backed off.
I also use Sprint right now just for unlimited data as, while I was in the hospital, I used over 400G in one month. It's not uncommon for me to listen to a recorded web conference while driving instead of the radio. Data Data Data. T-Mobile isn't great in this area, not bad though, Sprint is peachy though. Both the only unlimited options but every carrier is area dependent. I only use WiFi at work or at home.
Basically, it comes to the point of, if I can do work on my tiny phone instead of lugging around my laptop from meeting to meeting, why not. I've held meetings with it through networked projectors. Basically, I USE the device probably more-so then the average bear. Honestly, making phone calls is one of the least things I do with it.
Honestly, their super-thin phones today are a pain in the backside. I could not even use it without my large case on it. It slips out of my hands. Make a 'normal' sized device with a massive battery and I'd be more interested.