In the UK, SIMs tend to expire if not used for 6 months or so. If you put a spare phone in a draw and forget about it until there's an emergency, the SIM probably won't work. You won't be able to transfer a SIM from your main phone if it's been stolen. (And if getting water-logged is a threat, I'll safeguard my main phone as carefully as I would the emergency phone. No reason why what affects one won't affect the other.)

I'm not arguing against having two phones. By now, most of us have upgraded their main phone at least once, and so have an old phone they can use as a spare.
_________________________
Quality is addictive.