Okay, here's a real life example of not having purchased an additional warranty. A couple years and 20-some thousand miles ago I bought six Michelins for our dually. I turned down the road hazard warranty @ $30 per tire. On inspection yesterday the tread is separating on a rear tire, meaning I have to buy two new ones. Can't have one new and one old tire on a dually so they pro rated both tires. The pro rata price comes to about $240 for the pair, which I'll pay for myself.
If I had bought the road hazard it would have cost 6x30, or about $180. That's pretty expensive insurance for $60 worth of coverage!
BTW, those are the most expensive tires I've ever purchased for any vehicle and the only failure I've ever had. Who says you get what you pay for? Not so much, these days.
'11 Eagle 320RLDS
'02 F350 PSD Dually 4WD
DW's Ride, '13 Expedition