Originally Posted by Knudsen
This is true of either brake design. As viewed from the left side of the bike, the wheel will want to rotate CCW around the point where the pad grips the rim or disk. The rim brakes are also disk brakes, folks, using the rim as the disk. So if you move the pad (caliper) about, you can get the wheel to want to rotate up down, for or aft. It's a product of the position of the pad, not the brake type. Geeze, didn't you guyz have spiralgraphs?
Rim brakes and discs grip the disc (be it the rotor or the rim) at different radial points, and disc brakes exert a much stronger force than rim brakes. Rim brakes are not capable of producing enough force to eject a wheel and don't force the axle downward. Discs are and do.
That doesn't mean that it's a big risk if you take proper precautions. According to the linked article, disc brakes certainly ARE capable of ejecting a wheel even with a properly-tightened QR. Still, the liklihood of this happening is very small if you follow the proper precautions. The issue is real, but I don't think you're taking your life in your hands with discs, though, especially for road use.