Back to the OP's question (really two questions);
Based on my singular experience with a pair of Ncore NC1200 monos, the one sound quality based reason I can think of that somebody may like Class D over (high quality) Class A is low noise. The Ncore NC1200 amplifiers were dead nuts quiet. My Class A Clayton amplifiers are not as quiet as the Ncores but they are certainly no noisier than other Class A and AB amps I have owned, quieter than most, and quiet enough that noise is a non-issue. Sonically, the NC1200 amps came close to the Claytons in several respects such as having a nice tonal quality and full, powerful bass, but even in those strength areas, the Class A Claytons offer a bit more realism and enjoyment. In some other areas, such as high frequencies and soundstaging, I find the Claytons to be clearly better. With respect to sound quality and listening enjoyment, of the amps I have owned, I would rank the NC1200 amps about fourth.
So my question is, if it weren't for efficiency & power, would there be any reason to own a Class D amp? Do they beat Class A in any other categories that count for sound quality?The obvious reasons to buy a Class D amplifier are power per dollar, efficiency, and form factor (i.e., small). They should also work well in powered speaker implementations. From a sound quality viewpoint, some, like Guido who posts here, have had good luck with cutting edge designs from Rowland and Stereophile certainly gushed over the Theta Prometheus that uses NC1200 modules with a linear PS. However, even the Mola Mola that 6moons praised has received some responses that it is not quite ready to compete with the best.
Based on my singular experience with a pair of Ncore NC1200 monos, the one sound quality based reason I can think of that somebody may like Class D over (high quality) Class A is low noise. The Ncore NC1200 amplifiers were dead nuts quiet. My Class A Clayton amplifiers are not as quiet as the Ncores but they are certainly no noisier than other Class A and AB amps I have owned, quieter than most, and quiet enough that noise is a non-issue. Sonically, the NC1200 amps came close to the Claytons in several respects such as having a nice tonal quality and full, powerful bass, but even in those strength areas, the Class A Claytons offer a bit more realism and enjoyment. In some other areas, such as high frequencies and soundstaging, I find the Claytons to be clearly better. With respect to sound quality and listening enjoyment, of the amps I have owned, I would rank the NC1200 amps about fourth.