I don't think that the harness is that important, BUT I do think that the "baby set" is great to prevent them from falling. It is no longer necessary when the child can safely get up and down from tripp trapp themselves. My now 2 yr old has dwarfism so can't climb big steps (eg, she can't get herself onto the sofa as the sofa is as high as her nose!). Because of that, I'm still using the baby set with her. My older daughter was safe (enough) without the baby set at that age. Still had a couple of falls off it, mainly due to her fooling around.
Never seen the playtray before but have to admit, that is brilliant! We use our tripp trapps at the dining table so don't "need" it, but it is so well designed!!