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I think Rally drivers are allowed to race with the windows up, so the full face doesn't give a huge advantage?
HANS is great to have, but works only with dual should belt style harnesses (4-point, 5-point, 6-point) and is an expense a lot of people just choose to avoid. They are not mandatory in club style racing. It is a great device that prevents your neck from snapping forward violently in a hard front-on collision. It requires the helmet to be HANS compatible. It tends to limit your head's range of motion a good bit. If you have a high butterfly backed seat that tends to matter less as the visibility isn't there anyway.
Basically people don't run them often at the club level because of the cost, and likely-hood of that type of crash at those speeds is la bit less.
For 2012 SCCA requires a "HANS type" device. (I think that they actually just called out SFI specification 38.1)
It was my understanding that NASA had required the HANS as of 2010 or 2011.