[thejeff]

The only Spell song I've been able to make good use of is the fatigue one (Soothing Song?). It's great for big battles. A handful of cheap mages spamming that will keep your battlemages going as well as a communion. But you need critical mass. It's not useful with just a couple mages.
It's also good for keeping the troops going, but trickier to deploy, since the range is limited and it's hard to keep the mage near the troops.

That's my problem with the other two. The morale one is nice, but I don't think repeated castings stack. It might work against morale drops due to fear, though.
Healing would be really good if you can arrange casting mages right behind the front line, but they rarely stay in the right place long enough to be effective.

If someone has better ideas how to use them, I'd love to hear it.