A/B speaker switching - advantage?

What is the advantage of having A/B speaker terminals, besides being able to swap between two sets of speakers (maybe for sound quality checks)? Is it beneficial to have two sets of speakers connected to the terminals and have both A and B speakers on at the same time? I know you can connect a powered sub high-level inputs to the B terminals, but what if I expanded my current bookshel/powered sub combo with floor standers, and had the 4 speakers on the terminals and the sub connected to the pre-outs, would this have a negative effect on imaging/soundstaging?
I used to have Nikko Alpha amp that had this capability.
It's a possibility to have speakers in different listening rooms.
Ditto above, great for easy listening in the home, for parties etc. Being able to cut one off and the other on makes for a happier home at times in my house.
Speaker load may be a problem for the amp which could adversely affect it's performance. Read the fine print to make sure your amp can handle the speaker load you plan on giving it.
Regarding multi-room - the house we bought came with speaker terminals in the basement and the living room. I don't know what quality the wiring is in the wall though. There is also the problem of having my CD collection strewn throughout the house like leafs blowing in the wind. I prefer a system like Sonos or similar for multi room, but of course that is much more expensive...How does A/B switching differ from amps that offer "multi-zone" specifically, such as the NAD C725, does this just mean you can play two different sources in two different rooms?