"Specifications (distortion, noise, etc.) of preamp-processors and power amps are usually a bit superior to an A/V receiver

Good point and if I start from scratch it would make sense. However, many people think about adding an amp to already existing receiver. Even it's a very good amp it sill has distortions that will be added to whatever the receiver has. So one can end up it with the power that only a very small fraction is used AND more distortions.