Speaking from our humble experience as a capella singers, there are plenty of fine tunes that sound great without accompaniment... and it's not that hard to write a tune with those same qualities. Mike (the hubby) has done some fine tunes for C Fox Smith poems - both with and without backup instruments.

So it is more the nature of the lyrics and their presentation that determines if you want to go a capella or not. Realizing, of course, that your voice is the prime instrument that makes it a song rather than a poem.