Why would anyone want class AB amp when class A always sounds better ?

It's all based on one's perceived reality. 

Oh puhleeeze, everyone knows that Qobuz streaming in HR through a  Hybrid-Digital Purifi Eigentakt amp sounds better than a record player through A or AB.  C'mon out of that dark ages cave you analoggers are in...

There have been only a handful of true class a amplifiers made it takes a lot to make one and very few have ever accomplished it. In solid state amps only a handful and in tube only a handful also they are incredibly difficult to make and unreliable due to heat generation because of the 10 percent efficiency. So most of the makers if not all stopped making them. Now most makers build an amp to play class a to a certain watt level then switch to ab mode because of this and this results in the amplifier still sounding like a class a amp because it is at low powers.

Miller didn't say a bad word. If he wants to sound right - fine.
Going too far into the subjectivity will get us nowhere.
I mentioned D'Agostino class AB amps as being more than competitive.
Maybe his class A amps would've sounded even better. I bet, he didn't want huge size and heat, so he opted not to do it.

I can chime in on this. I owned a Plinius SA-103 @ 220w into 4ohms which is class A biased driving a pair of Legacy Audio Focus speakers @ 95.4 db sensitivity. I always felt like it lacked power. Indeed, there were certain passages where I would drive it to clipping. One of which was Loreena McKennitt’s vocals on the Dante’s Prayer track from the Book of Secrets album. I decided that was unacceptable and that I needed more power. I sold the SA-103 and purchased a SB301-MKII class A/B which is 470 watts into 4ohms. The difference was amazing. I had all of the great Plinius sound and none of the "lack of power" issues. The bass was driven with much more authority, the highs were crystal clear and well defined. I wasn’t quite sure what to expect but now I wouldn’t trade my amp for anything. YMMV but in my experience the additional power of the class A/B amp trumped the class A hands down.