Typed this late last night, but TR went offline and I couldn't post it:

If both sides of the suspension compress equally (you hit a bump with both wheels), the bar moves, but it does not TWIST. It simply rotates in its bushings that mount it to the chassis. (and if you happen to have poly bushings that aren't properly lubed, it'll probably squeak like a bitch... but otherwise you'd never know it was happening) It's the TWIST of the bar that induces spring rate that affects the suspension. No twist, no effect.

Remember, a swaybar is just a torsion bar with a lever at each end. Torsion = twist.

Now, a REALLY stiff swaybar will affect ride quality in a straight line. It does so whenever you hit a bump with one wheel. The shock of the impact on that one wheel, instead of being absorbed by that one corner of the suspension, is transferred to the other side of the car by that bar that refuses to twist because it's so stiff. But, as long as you're hitting the same bump with BOTH wheels... still no effect.