Don't do it, way overkill. While doing DH/FR mountain biking on the otherhand, I would never ride without one.
This train of thought is where my question came from. The hill doesn't care if you eat it at 30mph on a bike or 30mph on a snowboard. I'm not clear on why FR bikers won't wear anything but a full face, but only a few snowboarders consider it.
I guess it could kill the mood, like wearing too much gear to longboard. Something about the freedom...
That being said, I picked a Bern Brentwood for the time being, and am quite happy with the choice. I'm more likely to ride a snowboard hard than the FR trails. Next summer I'll look into the full face if I'm interested in hitting bigger stuff on the bike.