I've starting wearing a helmet in nets, where I'm facing our quick bowlers on a bouncy indoor track in bad lighting after I cracked my eye socket off a top edge a few years ago. I still can't stand wearing it though.
But outside, on a slow low pitch against dibbly dobblies? No chance. Very few competent batsmen at our level wear helmets.
As for the "why take the risk", there are lots of things we do that are riskier than batting against slow bowlers without a helmet, and wearing a helmet definitely impinges on my ability to both enjoy the game and to score runs.