I'm going with the NHRA on this one. I know they are not 4WDs, but the concept is the same. No hemet under 14 seconds, in a full topped vehicle. Helmets are designed to absorb impact. "bumping your head" on a padded headliner doesn't count as impact, it's just annoying.
Off-Road the same idea applies. A light to medium modified vehicle, with a full structure and top will protect you fairly well from whatever the world throws at you at normal 4WD speeds.
If you are driving topless and doing wierd stuff, like competition rock crawling, or mudding competitions, have a highly modified vehicle, or are in a speed competition, I can see the need for a helmet.
On a MC, on or off-road, you bet.
Same with ATVs, caged or not.
I'd like to see these sources the OP is talking about. I haven't heard or seen anything from any of the Off-Road sources I watch, about helmet use.