Here we go again!
Right, to begin with, I wear my seatbelt at all times except when I need to go to the lavatory. I believe most people do. However, in my book that means that by the laws of averages when any unexpected turbulence is encountered there may be pax walking to or returning from the lavatory. If the FD switches on the seatbelt sign I will respect it, but within reason. By that I mean that depending on how long they choose to leave it on, there will come a time when nature will take precedence. (Btw, my experience is that U.S. airline crews often tend to leave the seatbelts sign on for long and unecessary durations - thus reducing the effectiveness of it). And, right or wrong, many pax have a genuine need to go for walkies to relieve the agony of being squeezed in like sardines when flying Y class. Bottom line: inevitably, on longhaul flights there will always be a risk of injuries when unexpected turbulence hits.