I don't think music and lyrics in a merged form is any greater than music by itself. Have you listened to classical music? Jazz? I mean, history has shown that music is more than able to hold it's own as far as artistic expression is concerned.
It greatly depends on what the composer/artist want to express wether a melody in the form of a human voice is needed or not.. and when it's used, the melody, the contour of the line and how it flows through the harmony, the intrinsic vocal quality (speaking for myself), not the lyrics is what's important.
Also, I am not too found of an artist supplying meaning to a music piece in form of lyrics, as I prefer to derive meaning from the piece by itself.. but again, that is just my viewpoint..
I am not arguing though that poetry and music can't be merged to a beautiful co-existing artistic expression, only that music is perfectly able to stand by itself if it is composed without poetry/lyrics in mind.