I wear a light weight and well ventilated helmet most of the time but have no illusions about how much protection it will give my melon in anything more than a minor thump to the noggin type accident.
That said, I have been thumped on the noggin a few times without a helmet on and it would have been quite nice to have been wearing one.

The helmet in discussion here looks like it would make me so overheated I would probably pass out half way home and hit my head.