It might always be easier to beg borrow or steal someone else's idea, I don't know if it's ever better, than creating something new. The example (that one of the Jacks) used may be more effective but that isn't the reason that the majority of parodies are made. They are made because people who think up the words don't think they are able to go the whole hog and create a new tune as well. The point was well made that the life of the parody tends to be shorter than the original. Given some of the effort that goes into the best of parodies, they do deserve a longer life. The best songs are a delicate balance between the words and the music and are best left alone.