How important are humidifiers for Acoustic Guitars, really?

Hi,

I've picked up a very nice, 2004 12 string and want to keep it in tip top shape. I'm not an Acoustics guy so i really have no idea of the importance of humidifiers in keeping them in good order.. Are they essential? How often should a guitar be, say, in it's case with the humidifier. All the time or once every while if the weather's been dry?

I live in Melbourne, Aus so bonus points if people can say how necessary a humidifier would be for this location. Melb is like 60m above sea level.