Sure it affects the prequels more but it also affects the Vader/Luke plot in ROTJ, particularly Ben's explanation to Luke.
Also it affects the way people treat Luke. If it was common knowledge that Anakin had turned to the dark side and become a sith (and everyone would have found out especially amongst the rebels) then they would be very wary of Luke. However if Anakin is still remembered as a great hero of the old republic then they would be far more welcoming to Luke.