Poodles said:
every racecar I've seen ha had them bolted to the frame...
the harness bar is there to keep the harness on the level with your shoulders, if it's bolted straight down to the frame it will crush yoo in an accident
I did some looking around and realized that we might not be talking about the same thing. I'm talking about cars with a full cage, where the welded in harness bar should be used as the mount for the harnesses. But I found something that brought up a good point to support your side, that a lot of harness bars aren't really designed to take the loads in the event of a crash. And I'm sure most harness bars aren't really designed in that sense, I mean they're designed to fit, but I doubt that many people are actually doing real engineering design on them. So with a harness bar, Poodles is right, it's better to use it as a guide and attach the belts at the seat belt mounting points. But with a cage, a wrap around harness is still the best. One of the big reasons for this is the shorter length of belt in tension will allow less stretch in the event of a crash. This is also suggested in some rulebooks that I looked at real quick where they say to mount the belts so the shortest possible amount of belt is in tension.
But everyone else is right about the 4-point belts. They're junk, it's more for showing off and saying "look at me, I have a race harness in my car" than it is for actual safety. I got one when I bought a seat from some guy, had it in the car for one event and took it out as soon as we got back and put the stock belts back in.