You can get scratches on mbp even if you are super careful. And if you drop it a dent will appear.
I remember going to the genius bar 2 weeks back and this guy next to me had this giant dent right at the top of the screen. Had he used a case his MacBook would have been dent free.
You say you can still get scratches if you are careful, but I must admit that (after 9 years of laptop ownership) I have never managed to.
And a hard case will not do much to protect a dropped laptop. There are plenty of threads here on MR where people have dropped laptops in hard cases, leading to denting that is so severe the laptop cannot be closed. Even if a hard case prevents external damage, the likelihood of no components being damaged is slim. Either way, damage arising from a drop is not covered by AppleCare.
You can be pretty sure of
never dropping your laptop if you:
1 don't carry anything else at the same time when moving it around;
2 put it in a laptop bag (or a sleeve inside a backpack) when you are carrying lots of stuff.