Even though you are only 12 ft off the ground, and probably feel like you are sitting on your tailgate while hunting, you should still wear the harness. If only because you might be sitting there, and have an owl try and land on you - personally I think when you hear about hunters falling out of their tree stand, weird stuff like that happens. I've had squirrels run over me (I jumped), owls almost land on me (I flinched), a tree limb fall almost on me (not that big), you get the idea.
I don't think hunters just fall out of tree stands, or fall asleep and fall out, some kind of event happens where the normal reaction is to jump, flinch, etc and that is when your harness saves your life. Or, a mechanical break down happens, or even worse, somebody sabotages your stand.