In 40 years of serious driving, I've had three items flip up off the road and hit the windshield with any force that might have injured someone. None would have hit me. One would have hurt if it did.
But in a LBC on an interstate, if ever that is the case, and with 18 wheelers on the road, and stuff flying all the time, goggles and a helmet would be a good thought for someone tall enough so that their head peaked up above the windshield frame.
For me, I'd stay off the interstates, and I'd probably want a good baseball hat on or something like that just as a precaution on other highway trips, however ineffective that might be.
I can't see a helmet or goggles in any regular traffic. Imagine having a helmet on and being slammed in the side by someone you didn't see because of obstructed vision. Or, imagine a real life case--a boulder that comes bouncing off the road straight through the windshield, or something that hits square in the face and not just at the cranium.
Wearing a helmet in any of those cases would be false security. I think it's the bigger things to worry about.