When I first got the tractor I never wore the seat belt...didn't feel I needed it because I never went too fast and did mostly road grading where the ground was already flat. To my surprise, I soon found out that the tractor has "no" suspension to speak of and that even little bumps shoved and jostled me around the cockpit.
As I started doing more off road work, and after reading this thread, it became apparant that the wiser thing to do is wear the belt, like I do in my car, all the time. My 55hp KAMA has ROPS and enough power to "keep on going" even after I hid a large object...rock, roots, but the shock jolts me enough to realize I better have my belt on. The other thing is that the cab is quite crowded with gear shifters and levers and coffee cup and thermos holders, gun scabbord, all set up for easy reach. But that has made it very hard for me (I'm no spring chicken anymore) to consider jumping out...I'd probably get snagged and get my butt squished.
Now I wear it all the time for all work I do except when re-positioning the tractor a few feet for backhoe work. I do pop it on if I happen to be on unlevel ground even when repositioning just in case....I don't wear the backhoe seatbelt because it's positioned aways behind the tractor and visibility is way better and way easier to jump out of. Plus, the tractor is stationary and I always adjust the outboard legs to keep it firmly planted.