Spare batteries require you to turn the phone off, lose signal, boot it back up, etc.
Just carry a small portable USB battery. Same capacity (or greater). Same weight (or smaller). Same size (or more holdable). Charges your phone - or anything else - while your phone is still live. Can even piggyback on the phone.
At first, I was with the "no SD card, no replaceable battery" crowd, and then I sat and thought about it. The SD card - that problem is solved and has no better solution. But actually swapping batteries is a pain in the butt compared to just carrying a small USB charger around. They do them lipstick sized, they do them flat, they do them inside the phone case, they do them piggy-back, they do them huge for inside your laptop case, etc.
And if you're already carrying a battery, and presumably a case for it, alongside your phone, you've lost nothing but gained the ability to increase your battery life outside of even the maximum daily range without having to open any covers, drop your SIM card, pause your download, turn your phone off or anything else. And you can charge BOTH back up from the same charger (rather than having to swap batteries, charge in-phone, swap batteries, charge in-phone, etc.).
I have to say, the battery argument is now weak. The only remaining problem would be death of the battery entirely, by which time the phone is obsolete but still a £20 repair at your local phone-repairers would see you right for another few years.
And you can charge up your friend's phone when theirs dies too. Unless they use a stupid Lightning connector. In which case, stuff them... make them buy their own overpriced adapter cables.