I've been cycling for 50 years. When I got a job where I could start riding to work, I carried an inordinate amount of stuff, including full rain gear, all the time. Gradually, I started leaving most of it behind. The reason was that I developed a better sense of the actual risks of breakdown, and the resources at my disposal should something happen, based strictly on my own circumstances. This is also kind of how I live my life.

The people I know who've had breakdowns tend to be physically a lot stronger than me, and more "adventuresome" in their choice of riding conditions. For instance I expect my mountain biking friend to have a new story of destruction every few weeks. In some cases, I suspect that neglect or lack of maintenance skills played a role.

I bring more spares and stuff when I go on really long rides, or if my family travels to some remote place with the bikes.