Please note that what follows is only MY opinion...
Harness bars are fine if you're talking about autocross, where most of the track is done at sub-50mph speeds. In the event of a "whoopsie," you're talking about taking out a bunch of cones, even sliding completely sideways, but that's about it. When you start talking about open-track events, though, speeds are up dramatically, and the runoff in the event of a "whoopsie" is considerably less friendly. T5 at Road America is arguably the slowest corner in the NASA Midwest region, and that's sitting at 46-48mph. T11, "The Kink," is one of the fastest, with average speeds well north of the 100mph mark, and is flanked by concrete mere feet off the edges of the track. Coming into T12, "Canada Corner," you're above 140mph in a Mustang entering the braking zone. If you butcher the corner, and go off into the pea gravel sideways, you WILL flip the car. What protection does the harness bar offer you at that point? With a 3-point and stock seats, your body can slump sideways in the event the roof comes down. With 5/6/7-points, your head becomes the new roof support. To me that's just not the wisest thing you can do.
To offer a glimpse into my decision process when it came to safety items, it kind of went like this:
1) I'm turning lap times faster than 50% of the race groups in the region.
2) Based on the above, I want to run a head-and-neck restraint in the event of Bad Things happening.
3) Head/Neck restraints require the use of 5-point belts to function properly.
4) 5-point harnesses require a race seat and a raised shoulder anchor point to function properly.
5) A race seat requires, at a minimum, a 4-point rollbar for safe installation.
Ergo, when I moved from stock, I went to a harness, seat, rollbar, and a head/neck restraint as my safety package. I have NOT regretted it, at all. I feel completely connected to the car, feeling things I never would have without the seat/harness, and the DefNder head/neck restraint gives me a bit of peace of mind with the added impact protection that it gives.
If you're going to track your car aggressively, then this is a "must do" in my book. Don't think necessarily of how any piece of safety gear will help you drive, think of how it will help if it all goes suddenly wrong.
3-point belts, stock seats, and airbags actually make a HELL of a good safety system, but not as good as harness/seat/HNR/cage. They are safety systems, though, not just individual pieces. The real decision is whether you feel the stock safety package is adequate for the driving conditions you're putting the car into. In all cases, think of the worst case scenario (140mph brake failure, sideways into the gravel, multiple flips), and then manage your risk accordingly.
