The 'random deaths' are complications that elevate and change the story's direction around. So Ned's death starts the war in earnest. Robb's apparently closes it down but is actually more important in catalysing the creation of UnCat and showing (arguably) that Stannis and Melisandre's leech-magic has real power, which in turn helps catalyse their journey to the Wall.

Quentyn's death clearly complicates Dorne's allegiance to Dany, perhaps making an alliance with Aegon VI more likely (which will probably be a mistake).

George's wife has told him that if Arya dies, he's sleeping on the couch for a month