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

The Plinius amps of yesteryear allowed users to utilize an A or A/B button on the front of the amplifier to allow for Class A critical listening. The amp generally sounded its best in Class A, but A/B was very close.

I don't know if the current generation of Plinius amps allow for such user friendly options. 

Lower power consumption with most A/B amplifier designs. They tend to be popular in devices like DACs. Due to the nature of the Class A topology, they are very inefficient with electricity and typically generate a lot of heat. My Class A systems can only be run once or twice a week...otherwise my power bill will go up! 

Class A has a conduction angle of 360 degrees, whereas A/B is less.

In theory, this should translate to a more controlled and linear amplification of the signal. In most cases, Class A amps have more of a "grip" on the signal and can be "straight wires with gain." Everyone hear probably knows what I mean (see what I did with that spelling mistake) lol 

yep. So class A is superior if it is designed well without compromises.

Orpheus10, I don't have such a hearing as you imply, but my ear is quite sensitive to some aspects of a sound. 
Let's not transform this thread into tubes/transistors debate. Sure, ultimately tubes rule.
That's what we are trying to talk about - designs without compromises or almost without compromises. Heat, space, cost, look - this is all BS, just certain side effects. Done right it should always be full Class A design. And since Class A requires high level power supply, that's an additional advantage, that's where it all starts - with a power supply.

Audio Research amps run A/B and top of the line ARC are the best I've heard. I'm sure there are better, but they are the best I've heard.

I think balanced and unbalanced is also significant because balanced interconnects are not affected by long runs.

Anyone in the NYC area - Northern NJ area who would like to hear the differences, send me a PM.  I would be more than happy to show you differences in equipment - Class A tube, Class A SS, Hybrid amplifiers, Direct Heated Triode components, etc.  I'll even supply the pizza.  Bring your amp if you like!

Happy Listening.