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

Agree with the Benchmark stack being smooth, It is a smoothness that also has the most clarity that I ever heard.

I recently added a Gustard X26 Pro to my Benchmark HPA4 | AHB2 mono. It replaced the Benchmark DAC3B (still keeping it). The sound of my system was transformed to that of the Gustard. The HPA4 and AHB2 do not seem to add any flavor.

Comparing company X’s class A amp with company B’s class AB amp may not mean much. I bet even there are many class D amps that sound better than class A amps at the same price point.
I cannot think of apple to apple comparison, but how about these?

• Between a class A amp with another class AB amp at the same price point: if you find a class AB amp sounds better than or equal to any class A amp at the same price, why would you go for class A?
• Between class A setting and class AB setting from the same amp: if class A setting does not sound any better than class AB setting, what’s the point for the extra cost of class A setting?

For my Plinius SA102, I cannot hear any difference between class A and AB settings in most music. Only when the music is very demanding (more bass and dynamics) I can tell some difference, but still the difference is not day and night difference.

New Class Ds (e.g.NAD etc)..it's the future, like it or not. Have a listen, go on.I've had a range of A, A/B and early Ds over the last 50 years. Not going back away from D now - sound, size, heat, weight, WAF - all ticked.

The only thing that matters to me is the interaction with the speaker I'm connecting it to. Over the years I always seem to gravitate back to class A. It's a finer more complete presentation to my ears. The whole notion of everything on audio being black and white across the board is what makes the used market ripe with choices.   

Class A amplifiers don’t always sound better. It depends on the headphones or speakers you’re using. Headphones and speakers with superior transient response (able to keep up with and resolve complex music) tend to sound better with Class A. But not all are made equal. This means you could have a Class A amp that performs worse than a Class AB - although, if designed well and without compromises, Class A is the gold standard.

This is a great resource:
https://circuitdigest.com/tutorial/classes-of-power-amplifier-explained