Are DAC pins useful for anything besides working with audio?
They are incredibly useful.  They are essentially how you deal with the analog world.

For example, I'm using two of them in a project here that uses a frequency generator and a couple of op-amps to create a VSWR bridge, allowing me to figure out the resonant frequencies of an antenna...  The parts with the arduino cost less than $20... Buying something that does the same thing is about $300!

So, yes, they are very useful. :)