I always use batting because I like the soft look which it gives to a table runner. I am not as picky, perhaps, as I should be, and I use whatever batting or batting scraps I have on hand.
Without batting, a table runner is just a piece of cloth with a backing. I think I could achieve the same look with cutting a piece of fabric and laying it on the table by itself -- sort of a tablecloth which doesn't fit the table!