The mergering of several plants together provides a larger root mass, and alows the plant to better utilize nutrients / water
However when grafting your exposing the plant to potential diseases, Now if one gets a diseases they will most likely all get it. Multi-graft fruit tress are prone to these problems as such their almost never used commercially
I love grapes when I can beat the birds to them . . .
The wild ones are weedy hear, but rarely fruit since they are never pruned, but when you luck out and find one their out of this world