Our main concern is ... not needing a spare because ERS can usually bail you out without one ... but what if you can't get enough bars on your phone to get ERS?
Our Winnebago Itasca came with a full spare and the one time that we could - and were able - to call ERS to change a flat of course they did not have a spare along and could not have repaired our tire on the spot either ... so the spare from our RV was quickly installed by the ERS technician and off we went.
BUT, BUT, BUT ... what about the situation somewhere on the open road - or worse yet off the open road - in which your cell phone cannot "get out"? You might want to have your own spare (and the equipment) along then ... and be able to somehow carefully and slowly struggle to change it yourself or have a non-ERS good Samaritan magically come along to help install your own spare. ;)