I carried a second pair on every trip for years and then counted the wears, which I recommend doing before this argument rather than during it.
On a two week trip the second pair got worn on about one and a half days, and one of those was because I wanted to, not because I needed to. That is a third of my bag for a day and a half.
But the counting also showed something I did not expect: the value was almost entirely in the days it rained, and those days were not optional or predictable. So the honest framing is not "is a second pair worth the space", it is "how likely is it that my only pair is unusable for a day".
Which gives you a rule rather than a preference:
- Dry season, cities, paved: no second pair. Genuinely.
- Anything where the primary pair can get soaked and take more than a night to dry: yes, and this is the whole case.
The part that decides it is drying time, not comfort. A pair of leather boots that takes two days to dry is a much stronger argument for a spare than a mesh trainer that is fine by morning. Work out how long your main pair takes to dry before you decide anything else.