I had 3 in my N900 and regularly used to carry 2 with me because it barely got through half a day on 1 battery. How anyone could use that for a daily driver was beyond me. I also had a replacement for my E71 as after 2 years it wasn't lasting as well as it did. My C7 also needed a new battery after about 2 years.

My N9 probably needs a new internal battery as that lasts about 2/3rds what it used to. I carry a spare external battery just in case for that. I've a dead Lumia 800 that I might pull the battery from and swap.

My Jolla thankfully seems to have awesome battery life. I'd not be too concerned with going two days without charging. However, at some point the battery life will diminish and there's always edge cases where some buggy app decides to drain charge. And as I mentioned earlier, charging by pedalling a bicycle is my weird usage case where a spare battery would be handy especially if I've used GPS a lot that day.