I regularly air down my 31s, generally to about 16-18 psi but have done 14 with no problems. Any lower and you increase the risk of slipping the tyre off the rim, although I've only seen it a couple of times at about 8 psi.
Aired down makes hard ground and bumps smoother and the tyres really grab hard obstacles like rocks and stumps. I have noticed that I've taken a few more chunks out of my tyres since doing it regularly but I do like to drive rocky stuff a lot and it seems inevitable as BFGs aren't too strong.
Give it a try, don't be scared of going low - you have a spare and a good compressor will normally re-seat a slipped tyre. I'd also recommend Staun tyre deflators for ease, I've just ordered a set of E-bay after using Pete's (Thanks
) in Wales.