I used to carry one 25 years ago. I needed it one time. I mean, REALLY needed it! Rear derailleur cable broke, leaving me in the 12 tooth cog. This was before cell phones were something you carried along on rides. I was 8 miles from home, and any route I took to get home included at least one 5 minute, 6%+ climb, which I am pretty sure I'd have had a hell of a time doing in 39x12. Broke the chain, rewrapped it somewhere mid-cassette, skipping the derailleur, and rode home.
Lesson learned - replace your cables before you think you need to, because they're cheap and it's easier than having them break on a ride.
These days, I carry a cell phone instead of a ton of tools. I have a tube, levers, patches, and a multi-tool, but no chain breaker anymore.
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"Don't take life so serious-it ain't nohow permanent."
"Everybody's gotta be somewhere." - Eccles