Arwin pretty much hit the nail on the head. My receiver doesn't recieve audio via HDMI, so I have to use Component. It does, however, pass audio through to the HDTV, so I can still use that audio. However, since the PS3 diesn't support output through HDMI and Optical, I can't choose to use my receiver or HDTV, I only have one option.
If your receiver won't decode HDMI sound, why do you route your PS3 through it?