Cantedvalve -
I have a 69 GT350 with the roll bar and shoulder harnesses. I rarely use them. Here's the picture from the owners manual. I think it says it all. At best, if you stop short, you get great posture - worst case your arms remain behind the rest of your torso. I have not had the inertial reel grab hold yet while driving.
That being said, I admit that the combo of the padded roolbar and harnesses hanging down looks really cool - and have had many a spectator comment similarly.
Doug