The idea of the singer-songwriter concept is that it stands for the most direct communication between artist/s and audience, most suitable for personal messages. The principal problem is of course that many poetic messages, however convincing, are either quite general or uninteresting "navel-gazing". In the first case, it is preferable to have a good singer - with a voice and training adequate to the style of the song - than one who happens to be identical to the author.

If we leave singing aside, the question is still about lyricist and composer. The type of person we are thinking of is primarily a poet; let us assume an excellent one. Let us further assume s/he is a mediocre composer - should s/he also write the music or leave it to a specialist? The advantage of the first method is not so much the increased "authenticity", but that the author can simultaneously tweak both the lyrics and the tune until they fit.

Excellent lyrics can do with music that is merely good, and vice versa. If however either is below some line, the song will inevitably fail (in my ears), however good the other. My advice to poets: cooperate closely with a trained composer who understands your poetry; if you do not find any, improve your poetry. Ingenious multi-talents are exempt, but then they are not likely to ask us for advice.