Battery:
Carrying a spare battery extends the usable chare time. Since you should never drop a lithium battery below 40% charge (see
The Care and Feeding of Lithium Polymer Batteries), you get 60% from the first one plus 60% from the second one, or more than a full single battery charge.
Swapping batteries every month also extends their total life. (I have 2 original batteries for my Motorola V-551 that are 11 years old. I'm still getting about 95% capacity from them.)
If the battery goes into thermal runaway (and we've been seeing a few posts on overheating batteries recently), it's easy to pop off the back cover and drop the battery out, get upwind of it and if it burns, it burns. (Most replaceable batteries sell for $15 or less - not a huge price compared to the price of the phone.) If the battery isn't removable, you drop the phone, get upwind of it and watch it melt. (Thermal runaway isn't covered by most insurance plans.) If you don't believe me, ask Sony how much their burning batteries cost them. (I'm estimating at least 8 figures in US dollars.)
SD Card:
Of course having one means that we can carry gigabytes of music, movies, etc., in a shirt pocket. Most people would be hard pressed to fill the internal storage on a 128GB phone, though.
It also means that if your aboot - the first code that runs when you turn the phone on - gets corrupted, you put a bootloader on an SD card, put the card into the phone, turn it on and you can flash a new ROM and fix the problem. No external SD card and you need a box of the type that initially loads the ROM into a newly-manufactured phone. They're not cheap and you have to know what to do with it. Or you have to pay someone who has bought one and learned how to do it. Oh - and you lose everything on the internal storage of the phone. Booting from the SD card won't do that, so you can recover all your data.
And if you store all your data on your external card, and things get deleted somehow, recovering files from an external card isn't just trivial, it's boring. If you sit and watch the recovery happening, you'll probably go crazy - hours and hours of staring at your screen saver until something happens. Setting thing up to do the recovery takes 5 minutes - if you're slow. Recovering from internal memory? It can be done, but not easily.
Want to run more than 1 ROM (multi-boot, the way you can run a few different versions of Windows and a few different Linux distros on the same computer, choosing which one to boot to)? A custom recovery and an SD card to hold the images of the different ROMs. (I can boot my Note 3 into a Note 4 port at the moment. It's the only ROM that interests me, but back in 4.3 I had about 5 different ROMs on the card. [Someone come up with a Note 3 pure Linux ROM? Please?])
The cost of making the battery removable is ... well, non-removable batteries cost more, so the cost of a removable battery is negative - it costs less.
The cost of an SD card slot? If it's internal (you have to pop the back cover to get to it) - probably about a dime. Maybe as much as a quarter. (Remember, these ports that you can buy for 98 cents shipped from China, the manufacturer is buying in million-unit lots. There's a discount for that.)
So if it costs $2 more for them to make a phone with a removable battery and an SD card slot, and they want $10 more for the phone, I'd pay it. Even if I were buying the cheapest phone made.
If phones keep going no SD, non-removable battery, until there's no such phone, my last phone will probably be a Note 4 or a G4. Since I'm 73, that will probably last me the rest of my life. But I wouldn't want to be starting out now, facing a future of iPhones running Android (that's what they're doing, except that Apple - controlling all phases of the phone, hardware and software - does it better). I just hope that
Project Ara bears fruit.