uurrr...
I haven't made a point yet, just asked a question. And I don't think it was a daft one.
The helmet-in-all-circumstances types always suggest that because you can't tell when an accident will happen, and what sort of accident it will be, you should always wear a helmet. You can't risk assess head injuries out of mountain biking, so you should always wear a helmet.
But in buying a helmet, you are making that risk assessment: "I ride a lot in summer, and not that fast, so I'll be OK with this uber vented xc lid with no chin guard"
Unless you wear the highest protection helmet you can find (hands up if you do?), you've made a risk assessment about your likehood of injury and giving up some protection for some comfort.
That's not stupid, deluded, irrational or selfish. It's a decision, your decision, and you went through the same mental steps to get there as someone who decides to give up a little more protection for a little more comfort.