You know, Jake said in his DVD "Million Miles Away" that he has "lyrics" for a lot of his songs. It seems to me that having lyrics to a melody would help you create something far more cohesive. For example, in a typical song, you have a verse and chorus sort of structure and that lends itself to a "theme" for your tune. It gives you something that runs from start to finish and makes it much easier to build all the elements of a song. You can create a nice "hook" - an expression used to describe that catchy part of a tune that is easiest to remember or just "grabs you" and keeps your interest. A chorus is often like that.
Building a song around a melody rather than a riff gives you a much better over-all tune.