Here's why . . .
What you're really trying to do is turn the engine torque reaction into something useful - like re-planting the RR (that wants to go light from the pinion trying to climb up the ring gear). Basically, you're working the engine torque reaction against driveshaft torque with the idea being to end up with very little change in rear wheel loadings. This is really a chassis roll phenomenon.
Drag bags only "sort of" address this, mostly by preloading the RR so that when the load eventually does come off of it you hopefully end up about right. That ends up being a bit of a guessing game, while the antiroll bar approach only reacts as much (or as little) as the situation requires. That said, there might be a little advantage to be had by preloading an antiroll bar, but that's something to tinker with for a specific car after adding one rather than being an important reason for choosing an ARB in the first place.
IMO, an ARB is going to be a lot better about keeping rear axle roll steer down to a minimum. Having to anticipate and compensate for tail-happiness is going to steal some of your concentration away from your main job of winning the race. I can't put a number on this, but it's never going to help your performance.
With either method, chassis torsional stiffness matters, as does making the front suspension roll stiffness as low as practical. Any "twist" that you lose along the way from the engine mounts to the rear ARB is lost load as far as planting the RR is concerned, up to the point where you pull both front tires.
Incidentally, this is all related to the reason that soft front springs and disconnected or removed front sta-bars work at the strip - reducing front roll stiffness has a similar effect as adding rear roll stiffness.
Assuming that the rear suspension geometry and springs are at least somewhere near right, there is no need to add any pure vertical rear stiffness, which bags also do.
Norm
