This is totally anecdotal, but it's what made me willing to consider the merits of non-removable when I started reading about them. But I think the best example of the case for and against would be to consider the case for and against substituting Tablets instead of Phones.
So far my phones with removable batteries were the LG Ally (never swapped it), HTC Thunderbolt, Galaxy Nexus and LG G4. TBolt and Gnex both REQUIRED extra batteries, they struggled at getting a day's use in almost any conditions. The G4 we had the free extra battery and cradle, but don't think it was ever used.
So far my devices without removable batteries have been the Moto X '13, '14, '15 (Pure), Moto Z Force, Moto G '14, '15, G4 Plus 64, Nexus 6, Nexus 5X, Nexus 6P, Pixel, Pixel XL, Zenfone 2, iPhone 5s. The thing they all have in common is better build quality and better battery life than all of the devices in the first list. Most of that is because they're newer devices, etc.
My use case is also slightly weird, because I carry two phones on a daily basis and so some of my work/play processing is offloaded to one device or the other which reduces drain somewhat on both.
So far my phones with removable batteries were the LG Ally (never swapped it), HTC Thunderbolt, Galaxy Nexus and LG G4. TBolt and Gnex both REQUIRED extra batteries, they struggled at getting a day's use in almost any conditions. The G4 we had the free extra battery and cradle, but don't think it was ever used.
So far my devices without removable batteries have been the Moto X '13, '14, '15 (Pure), Moto Z Force, Moto G '14, '15, G4 Plus 64, Nexus 6, Nexus 5X, Nexus 6P, Pixel, Pixel XL, Zenfone 2, iPhone 5s. The thing they all have in common is better build quality and better battery life than all of the devices in the first list. Most of that is because they're newer devices, etc.
My use case is also slightly weird, because I carry two phones on a daily basis and so some of my work/play processing is offloaded to one device or the other which reduces drain somewhat on both.