Insurance?? Peace of mind??? Call it what you'd like. If you are sitting on your deck photographing your grandkids running in the back yard, then don't bother. Try explaining to your editor that every shot is out of focus because something was on your mirror messing with your AF. See how long you are in the business..
I had a 100mm macro that needed repaired. Free one way shipping + 15% discount on repair was about $45.
So where are you if/when you send a piece of gear in for the 'free' service, then the day after you get it back, while you're out on assignment, something bad happens. You've wasted your service money. You still didn't get the shot you wanted. Your editor is still PO'd. What did you get for your money.
The only way to guard against the scenario you describe is backup gear. And even backup gear is fallible.
I'm happy you got a 15% discount on the 100L repair. If you weren't spending $100/year and paying to service gear with nothing wrong with it, you could have bought a new one with what you saved.
I realize everyone views insurance differently. If it works for you, by all means, carry on. I'm just trying to put my point of view out there. IMHO, insurance is for stuff you can't afford to fix/replace yourself. I save about $800/year by carrying high deductibles on my home/auto insurance. In the 34 years I've been a home owner, I've had one potential ($5k deductible) claim. It came just about $5,000 for roof damage from a storm. Yep, I paid it out of pocket --- with all the money I've saved over 34 years of not buying more insurance . Let me see $800/year, lets round that down to $400/yr X 34 years, minus $5000 is... uhm, about $8,600 in my pocket. Which means I could have $8,000 in non-insured storm damage tomorrow, and still be ahead of the game.