Never ridden a cruiser bike before but I would still sport a full face helmet. I can't imagine going 70mph and having a bug, rock, trash, or even the wind hitting my face.
I got into a little debate about gear, not just helmets, with a Harley rider who is sort of a friend of mine. He said he didn't all the gear (full face helmet, leather/armor jacket, gloves) that sportbike riders needed because he didn't go as fast. I pointed out that we both went 60 - 70mph on the interstate. He said it didn't matter.
I offered to push him out of a moving car at 40mph with just his vest and half-helmet on and see how it felt. He declined. But he rides on the interstate in just that all the time.