I will go un harnessed up to 20' depending on what the landing looks like. Generally speaking I am super comfortable up to 15' however beyond that I get a bit worried. This comes mostly from my time bouldering (climbing short rock walls with just a small pad and some spotters). I actually really enjoy pruning small trees like crab apples, young red maples, jap maples, magnolias etc without my saddle; just me and my handsaw.
Depending on what type of helmet you wear it may not makes sense, if you wear a standard hard hat with no chin strap then it wontt do you any good in a fall anyway, so hey forget it. If you wear a climbing helmet that offers some side, back protection than why not. In most cases my hard hat/helmet is on to protect me from stuff thats falling, not from me hitting my head.
Thats my .02