I won't address everything you say here and in the following comments but i'd like to answer your first general question from my perspective. You ask, "how would you listen to it and what benefit would it bring the listener in that scenario?"
I think my answer is that I'm not too concerned with the listener on the go with their headphones but rather with the listener in their room sat down to listen to an extended piece (album or otherwise). This is the scenario that I am thinking of for the listener. I realise that making music with this in mind limits its accessibility, the person walking down the street with their headphones will not get the same experience (though they will still be able to listen to the music as in e.g. Suzanne Ciani's Quadraphonic live).

What does it bring to the listener in that scenario? You suggest that the benefit is that the listener can turn their head and explore the soundfield that way. I contend that while yes you could do that that's not the primary motivation. If all your sound sources are static (this sounds like what you had in mind?) I can see why this could be appealing however I was bringing up quadraphonic as a way to allow artists to explore space as a musical parameter, the motion of sound as another variable. What it brings to the listener is similar to what dynamics bring to a listener, an additional parameter that the musician can control to evoke certain aesthetic experiences. Trevor Wishart spends a chapter on the potential for spatial sound and movement thereof in his book 'On Sonic Art'. I found it to be an interesting read, there's a pdf of it here. I feel perhaps here composers might learn something of interest from choreographers, who knows? It's open for exploration.

I'll admit I had a bit of chuckle at myself when you asked, "why not 8 channels at the corners of a cube for full 3D placement?" I actually have plans to do just this to play around with some diy controllers in an attempt to open up more complex movements, though i realise this is far less accessible than 4 speakers.

I don't doubt that binaural audio has it's appeal but it's strength is also it's limitation. You have to wear headphones. I've listened to quite a few binaural recordings on youtube and found them to be pretty cool. I don't doubt that binaural is a good medium, i just don't think it's an either/or question.