Just to mention a different kind of variable that no one's mentioned yet:
I'm going for my first SR series this year, and each ride strains the budget. If I'd blown last month's 600K, for example, I wouldn't have been able to afford another one this season. (Hotel, train fare, missed work, etc.) I'd have been done for the year. So I err on the side of bringing more tools, having an extra tire along, and having lights with me if there's any chance I could be riding outside daylight hours.
So the consequences (not just the relative risk) of a DNF figure into my decisions too.
I also train with all that stuff aboard. I could say it's because I don't see the point of carrying less weight during training than during actual events, but mostly I'm just too lazy to take everything off.