NVIDIA and AMD with their triple display setups strive for game compatibility without requiring the developer to make changes to the games.
What you're suggesting would most definitely require developer participation. I think for this reason they ruled this out as the way to go because for a new technology to make waves it really needs to be as frictionless as possible.
Take 3D for example, what are the two biggest issues with that technology?
1. You need a new screen (120Hz or higher)
2. You need to wear the 3D Glasses
People are willing to buy a high frame rate display anyway because it's all around a better experience and it just works with everything. You don't need anything special to see the gains. But for 3D you do need to wear those pesky glasses which is a burden.
It's the same with 4K. On Windows it is having a difficult time because although the displays are now available Windows is still not playing great with those displays. The UI is inconsistent and some programs are so tiny on the screen you cannot use them while others scale perfectly.
Now to bring it back to the triple display thing. NVIDIA and AMD have both been hard at work over the past few years to make Surround and Eyefinity better. I'm an NVIDIA user and I can say that NVIDIA brought out a toolkit which maximises program windows to only the display panel they are in instead of across the whole desktop (filling all three displays) they centered the task bar instead of spreading that across all three displays.
AMD and NVIDIA worked hard on game compatability, they even got some developers to include native support for triple setups. Guild Wars 2 and Battlefield 3 come to mind.
But doing what you're suggesting would 100% definitely require developer participation and I think if they went that way only, not offering the current system as-well that Surround and Eyefinity would be seldom seen in games, it would be like Mantle or PhysX instead of how it is now where you can fire up almost any game you can think of and just play it in Surround/Eyefinity.
But that doesn't mean this idea isn't good, I think it should be pursued as an option for developers to make use of if their game fits. Driving games for example really don't need high details on the side panels and if a developer wanted to spend some time working on that I think they should be supported in doing so.