Ga requires them so I use one. Have gone through 4 different options until a settled on a very light Badass brand 1/2 helmet. One of the lightest on the market for reasons noted. I'm also a sports car guy and really love the open air feel. Before the SlingShot I restored a Triumph Spitfire and it was smaller and felt less protective than the Sling but required no helmet. Currently have a first gen Miata and again the only difference is an extra wheel and a A-pillar windshield. Is it safer than the two rear hoops...? Hard to say but we are accustom to the hard framed windshields so assume yes.
Now a will say that riding the SlingShot in the winter with a nice warm full face helmet is nice and a lot less funny looking worn in the SlingShot than the Miata ...