I use my Kubota mowing very steep slopes commercially - graded capped landfills, reservoir dams, etc. I've used both open and cab tractors, and often thought about the pros and cons of wearing a seat belt. The *only* con I can see is if the tractor rolls into water. When I mow the top of a reservoir dam, there is very short, steep drop into anywhere from 30-80 feet of water. Just being able to "get out" of the cabbed tractor would be difficult. I'm extremely careful along water's edge. Otherwise, I wear my seat belt on any slopes. Working on flat ground, I do not.
Life is full of risk opportunities, and an enjoyable life is one where the individual gets to choose. I live in NH where seat belts in road vehicles are not required, but I frequently travel into Vermont where they are. I once was stopped for a seat belt violation, and the officer gave me a lecture on safety instead of the $100 fine. I politely waited for him to finish, then asked him if I had chosen to ride my Harley for this trip instead, would it have been OK to not wear a seat belt?