I in the always wear a lid camp if I am honest only take it off when walking with the bike if at all. Had some issue with feeling too hot in the summer but got a more vented lid for the proper xc stuff ie meridas etc. Other wise I have a Xen.
I have seen my mate stack and snap his lid clean through on the bit above his ears and some of the vents after a drop went wrong, lid must have done something there.
Agree with the risk thing though I dont think that wearing a lid or pads makes you safe from everything but it helps in most cases I think. I know a chap who rolled his car as a teen ager and got thrown out of the open window as the car rolled and burned if he had worn a seat belt he more than likly would have died now he never wears a seat belt I just cant see that this is a good thing.
For me the pros of a lid out weight the cons and I have looked at this a lot look at journals etc on the web. so I choose to wear one, if you dont well I think your daft, but its your choice if some one stacks I'll always ask if they are ok, help if I can. The I wont help as you didnt wear a lid is shocking mate. By that thinking if you were wearing spd didnt clip out in time and fell and hurt yourself could I ride past thinking well cant use them properly so F88k em??
Either way hope we never need to use our lid or skulls in a crash related incdent!