I can think of three things, but in terms of performance are nominal and highly conditional: 1: If the source's output is a balanced signal and the 978's input balanced, you maintain a highly noise & distortion resistant signal between the components. 2: A potentially more secure physical connection versus RCA (negated if a locking RCA connector is used) 3: If the 978 is located in a electrically noisy environment (ie-PC nearby), a balanced cable should offer better noise rejection than say, a cheddar-cheese (poorly shielded) RCA cable.
In short, I don't think a balanced input on the 978 would offer much sonic improvement over a quality RCA connection; especially if the distance between components is short.