I got a 3GS as a gift from my sister for Christmas once. Had a slight crack in the top right but no big deal.I think you may be on to something here.
As you say, you're way more protective over your phone because you don't use any sort of protection. Whereas those who do use cases and/or screen protectors are more careless with their devices because the device is protected and less susceptible to environmental damage.
So maybe they balance each other out? Maybe it results in a very similar level and risk of damage for both parties.
Ps. Even though my phone is protected to the max, I still find myself being careful where I lay it down e.t.c hah
My sister is a case user and she mentioned when she gave me the phone that she had thrown it against a wall in frustration once. Hard. The case protected the phone she told me.
And there's the thing. She gave no thought about what she did at the time because her mind had already processed the fact that there was a case on the phone.
The fact that the case DID protect the phone only reinforced this in her mind.
I don't use cases. If I throw my ~$900 phone against a wall I am NOT going to have a working phone when it gets done bouncing around. Consequently, there is something in my mind that would go off and warn me.
Good assement of my argument.