A couple of advantadges I can think of. One, the chips will fall off the tool easier. Its nice to have gravity on your side. Two, the stress on the machine is similiar to using an upside down cutoff tool mounted on the back side of cross slide. Three, as you mentioned the ability to keep from crashing onto a rear shoulder. But where this really shines is on internal threading where you cut on the back side of the bore from the inside out
The optimist says the glass is half full, the pessimist says it's half empty. The paranoid in me says somebody put a hole in it.
Remember pessimists are at heart opptomists. They know things can and will get worse.