That question all depends on how much of an audiophile you consider yourself to be. I use my optical input on my G5 PowerMac to connect an external CD player into my system for use in Garageband, or recording (Digitizing Cds) that have issues being read by my G5s internal drive (have a few like that).
Having Optical Output is nice if you have high standards for audio quality coming out of your computer, and into recording gear, or high end audio systems that you may want to drive from your computer. For most people, normal Analogue cables should be fine.