Optical audio in consumer equipment is very limited. It seems like it should be high-bandwidth because it's optical, but it falls short. For consumer audio equipment, ordinary RCA cables and TOSLINK optical cables do the same job, and the optical cables don't have better bandwidth or let you do longer cable runs.
You get two channels of PCM audio, or more channels of lossy audio. If you're running stereo or 2.1, I think it makes sense to configure your Roku to send a stereo S/PDIF signal over TOSLINK.