Instead of listing ways in which songs are useless, let's talk about how to
use different songs. I haven't played a bard in a long time, and there's a
whole lotta new songs out there - so I'd like to know which songs will be
useful in which ways and combinations.
So.... as a starter,

1st Level - Chant of Battle:
The first (and for a while, most useful), it's sung in combat - but since
it's not harmful, it should be sung at all times during a young bard's
early life until they've maxed out their singing skills. It enhances
combat abilities for the Bard and everyone in their group, and can be
successfully twisted / stacked / harmonized with most other below-tenth
songs. It becomes outdated with the tenth level song: "Anthem de Arms"
(can it still be stacked with that, though?)

Talies the Wanderer