the only occasion I have met when the wearing of a seat belt was not good was in a boat yard, where the truck had a negative lift mast to lower boats to the water and lift them out. When asked the question, I was with a H & S Officer, and he agreed with me that, provided that a risk assessment showed that it was safer to be able to get out of a truck that had fallen into the dock, than to be trapped under water when wearing the belt, it was better not to wear a seat belt.
Otherwise, my advice as a trainer was for the operator to always wear the belt. If he didn't do so, he would be at least partly liable for any injury that occurred as a result of not wearing one.
Although I agree with general statements here, and I am a newbie at only 40 yrs experience, lol, I will keep an open mind as to when a seat belt might not be safe to use. However, accidents do happen even when you are moving the unit 2 inches. So, when is it not safe to wear a seat belt? Turning from the hips to see behind is not affected by the seat belt. Loading and unloading trucks is a definite time to wear it, no excuses. Up or down a ramp? Up the ramp and you can't see for the load means you must have a spotter. Can't agree there. In and out of the unit several times builds complacency and so is a must time. As I said, I will keep an open mind, BUT, I have yet to come across a situation whereas it is not safe to wear your seat belt in my limited time of 40 years. Just my 2 cents worth.
You are right Paul I was painting with too broad a brush regarding trainers. In fact I haven't met a great number of trainers and chances are there are many good ones like yourself. When I was active training in the Vancouver area I was frustrated that my competitors wore blinders and insisted that operators should always wear their seat-belts. Someone made a comment that the only time operators always wore their seat-belts was at a training session or rodeo. Thanks for the input.