I've been wearing a harness ever since I started using a climbing tree stand, something like 15+ years ago.
I will admit though with a home made stand that we had on the farm back home I didn't but that was back in the mid 80's & I don't think I had the wisdom back then I have now.
My first time in a climbing stand which came with the harness "just a belt around you really but it worked" well I didn't know what the little string was for, you know the one that tethers the foot base to the seat, well I go climbing up this oak using the harness like a friend showed me & all of a sudden the foot base dropped nearly to the bottom of the tree.
I think I was around 12' up hanging by the harness & holding on to the seat, took what seamed forever to get into the seat & it took even longer to get lucky to snag the foot base using the bow rope so that I could get back down "no cell phone back then."
Ever since then I swear by having a safety harness when I'm not planted on the ground & on my club if anyone plans to be in a tree stand of any kind they better be using a safety harness or they will be asked to leave, I've gone all the way to my sweet spot once with everything but my harness & just hung my climber at the base & hunted on the ground because of that possibility of me finding ground faster than I can handle.