"The moment the composer begins to create the musical verses of his song, he destroys our appreciation of the poem as poetry, and substitutes an appreciation of his music as song. This is true of even such exactly corresponding pattern of poetry and music as the endlessly repeated verses of a folk song. In fact, it is in my opinion the absolute of the song as a genre. ... As soon as we sing any poetry to a recognizable melody we have at that instance left the art of poetry for the art of music."
Michael Tippett, "Conclusion" to A History of Song, ed. Denis Stevens.
I disagree, but I am glad to have a view stated like this by a composer. The main dichotomy is between a view of song as song, a unified art form, and song as a hybrid; in the latter view, the music usually comes after and is destructive of poetic value, or at least antagonistic toward it.
Usually vernacular traditions are more geared toward fusion, and classical traditions more toward hybrid views. But Tippett is uncompromising.