The only reasons to eliminate user-replaceable batteries are to save cost, and maybe to profit from expensive battery-replacement services.
Those are legitimate reasons but they aren't the only ones. By making the batteries not user replaceable companies like Apple avoid a variety of problems. The most important consideration is that don't have to worry about poor quality batteries from third party vendors. This can cause all sorts of headaches including warranty claims, product image problems, counterfeit batteries, lawsuits, etc. Being able to maintain full control over the product should in principle result in a better or at least more consistent product. Another consideration is that by sealing the device up tight they don't have fitment issues where pieces come lose or break unexpectedly. The more parts that can move the more chances something will break.
I expect the battery quality aspect is even more prevalent in the thinking than people realize, but it's more than that and above.
Structurally, it's much more efficient to have hard joined pieces and be able to rely on the whole case for strength when you design. The S5, while a great phone, is not as rigid as it would be if the back was permanently affixed. Plus, the snap-off, slide-off, or other attachment methods take up precious interior space. Maybe not a lot, but there isn't much to begin with. The seals on any removable cover will also wear out much more quickly than a fixed seal. So to make a battery user replaceable, you have to make your device larger, less durable, AND take into account poor 3rd party batteries ruining the phone's reputation.
While replacing a battery is handy, it's not THAT much smaller than bringing a battery pack or using a battery-case, some of which DO have removable batteries.
To be honest, I respect what HP did in their latest round of laptops - made them slightly thicker and significantly increased the battery. Oh, and they have a headphones jack too. FML.
Assuming Samsung makes this stupid choice as well, this will be the first generation of cell phones that I DON'T upgrade to since...pretty much Nokia/StarTac era.