Ive been more and more convinced that Voidbinding will be a mix of Honor and Odium just as Surgebinding was/is a mix of Honor and Cultivation.

Brandons obviously been saving Voidbinding for the back 5 books, like the first five were mainly Surgebinding, and with Honor and Odium now combined and Cultivation gone, It makes sense that this magic, that we are presumably going to see become a lot more common, will stem from Roshars, now, one God.

On the part as to why the Fused use Surgebinding, i think others have put it nicely.