While there are lots of careless people out there, there are also plenty of careful people who for one reason or another dropped their macbook pro's or damaged them in some way due to impact. For example, what got me thinking about a case was the other day, I slammed on my brakes in my car and my MBP started moving towards the dashboard. If I had slammed on my brakes harder, or if there weren't books on top of my MBP increasing the normal force (and concomitant static friction between the MBP and carseat), the MBP would have hit my dashboard, potentially damaging it. Such an event would void the warranty, something I'm more concerned about than mere scratches or even dents.
You make a great point, however unless you're looking for a more slimline looking case such as those hard shells a well padded case such as the Incase bag (www.goincase.com) would greatly reduce shock especially in terms of the example that you used about hitting hard on your brakes.
Every situation will be different for many people. You may not find reviews on the web of tests being done to show a Macbook being dropped inside of the hard shell cases. Just my opinion, but I don't think many reviewers are willing to do "crash dummy" tests on their Macbook Pro as it's a high risk. The impact could still shatter the glass.