AMD is promoting it IIRC. If it became standard that adding another GPU would just work, that would be great. I think that is what they are going for with vulkan/dx12.
Well honestly it makes sense as long as you aren't limited by case form factor.
You can make smaller dies cheaper, and you are going to sell a lot more of them (mass market vs niche high end). It makes a simple upgrade path as well.
The killer is the games that don't support it, because then you just have half your system power unused. However with newer games like DXMD having 100% scaling it shows just how great it can be.
If all games supported it, it would work well. But with so many that don't it often is a waste of money. And because many games don't, many people then don't go for sli / cfx, which then reduces the payoff for developers to support it.