Maybe I'm just paranoid, but I always go the extra length to ensure absolute rock solid stability of my system. I don't ever want to encounter something where I experience a crash or glitch because of an overclock, because I would find that frustrating. That's why I go to great lengths in cooling, take the time to work my way through the process slowly and find a point of stability that I'm happy with. Stress tests are an integral part of that because they represent the absolute maximum/worst case scenario for the chip. If it can handle that, it can handle anything.
I have no idea why people even argue over this. When you're overclocking don't you want the assurance your system can handle whatever is thrown at it gracefully? Using your car analogy, you're essentially advocating removing the seatbelts and airbags, because any "safety margin" for your overclock is eaten up if you can't pass a stress test at your settings. So what happens when you suddenly load up a game that's incredibly CPU intensive and it maxes all cores and now your system is crashing? Whoops, guess you should have tested better.