Nowadays? Never, obviously. The odds of me breaking the phone are so large, and the tools I need specialized enough, that I need a new phone on stand-by anyway.

Nowadays? Never, obviously. The odds of me breaking the phone are so large, and the tools I need specialized enough, that I need a new phone on stand-by anyway. No point replacing the battery if I already bought a new one just in case I break the old one by trying to do something as weird as replacing a wear-heavy part. So it has fallen out of style. There is no market for spare batteries and using them as range extenders is not common use. It seems like an outlandish thing to do now. I think I went through three batteries on my Galaxy Note 2 (first Android phone) before upgrading. Apps dropping support became a problem... nothing wrong with the hardware at the time where I felt forced to trash it. Anyway, carrying an extra battery for long travel days was not a weird thing to do. I also remember non-tech people having spare batteries for Nokias (when they became more capable; not when the only use was calling your mom to say you were going to a friend's after school).