TD, we strap in for landings to cater for the odd heavy one, or the even rarer case of the aircraft leaving the runway. I don't know the stats, but I believe that a typical airman can stand up in an aircraft for every landing in his career and not get seriously injured. So, if I had a choice of either sitting down and strapping in to cater for a very rare event, or to remain standing and fight a fire or tend to an injured person, I know that I would choose the latter. This kind of decision is not in the manuals. Every case is different.