Throughout my life I have needed to or maybe chosen to wear harnesses in transport throughout my life because of my posture and to maintain a comfortable position. Despite the fact I can walk I am not very good at maintaining my posture, and certainly not good at remain still when I am in a vehicle or wheelchair breaking sharply.
I feel harnesses are one of those lifestyle choices disabled people make on an individual basis and demonstrates for example, people with cerebral palsy may have similar levels of impairment but may manage it in very different ways. While some people may think it is odd I wear a harness when people they perceive less able than myself do not, I do not think there are any right or wrong answers so long as the wearer is the one making the choice.
My only advise is wearing a harness is always to have one with a crotch strap to stop the harness from riding up or the wearer from sinking down a seat to a very uncomfortable and maybe dangerous position. Why not keep one a try yourself?