I don't hear of that many people going from LR to Aperture, but I can't recall any who have mentioned image quality as the crucial factor. Most often it's a preference for Aperture's UI, or more recently a bad experience with LR4.0 slowdowns. A few people switch because of very specific requirements. For instance the timing of the D800 release didn't allow LR4.1 to support tethering that body, and the delay has caused a few to jump or threaten to do so. There are others who send images to Aperture for certain features which it does better - custom books or in my own case for standalone slideshows.