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

As a direct example, some feel that the Luxman 509X class AB at $9.5k sounds better than the Luxman 590axii class A at $9k. Can’t get much more absolute-busting than that. Same company, both integrateds with same features, essentially same price, both current products.

Other than seeing the operational state of A or A/B on  a scope or test equipment, no one can "hear" such conditions.

A  "matter of fact" from the wisdom is needed  to clarify my unqualified babble based on taking 101 electronics decades ago. 

Clas A amps DO NOT always sound better.
They should in theory perhaps but this ain’t theory now is it?

Provactive silly question.
Of course not what you mean.
Who cares whatever that is?

Yes, mapman, done right they do. 
Gryphon, Accuphase, Pass, Luxman etc.
"Some feel" funny things, that's for sure.