It's not like it's that hard to just swap batteries to charge your spares. I've never really had to have more than one additional battery though. I would just connect my phone to charge in the car or at home, then swap in the spare if it also needed to charge before going to bed.
I've also used an iphone with a battery case, and even have a 10Ah external battery. If I ever lost track and the phone ever dipped far below 80%, swapping to a different battery case barely helped because of the losses in converting the 3.7v battery voltage to 5v and back. I've noticed the same inefficiency with battery bricks trying to recharge a phone on the go while also trying to use it.
Battery cases are really only useful if you can swap them out as soon as the case dies. Which is why I say battery cases are a crutch, because it doesn't really solve to lack of capacity IN your phone's battery (getting a cast). If you don't swap cases when the first one dies you're wasting energy trying to charge the phone battery and cover the power the phone is using to operate. High capacity battery bricks only partially negate the losses by brute force in the form of sheer capacity but are more cumbersome to use. Trying to pocket a battery brick in a way that still allows charging, without breaking the usb connectors, can be hella awkward, even with cargo pants/shorts.
Carrying 3 spare batteries or simply getting an extended battery for your phone is way easier, and cheaper, than trying to carry as many spare battery cases or trying to carry and charge from a battery brick on the go.
If phone manufacturers would actually provide external battery docking terminals that made battery cases act like parallel capacity without trying to recharge the internal battery, I'd be more inclined to not care about removable batteries. However, thinking long term, I would still care if the internal battery were hard to replace in a DIY sense, like the HTC One M7/M8 and very likely the M9.
Samsung has actually been in the habit of making standalone battery chargers for their flagship S and Note phones. I was just at Fry's Electronics over the weekend and ~$40 gets you an Official Samsung S5 battery and the standalone battery charger.
Motorola was also pretty good about making accessories like standalone battery chargers and desktop chargers. Sure I had to get them from ebay or amazon but I did my research and was able to get official Motorola standalone chargers. One of which was actually dual sided and capable of charging several different sizes and types batteries.