I agree with you. At this time, the advantages that I see for HDMI over optical are (a) transmission of lossless audio formats and (b) audio and video over one cable.
I don't have Dish, but I sure don't think that they are using the lossless audio formats or any audio format that an optical connection can't support. So, I'd expect the audio quality to be the same over either HDMI or optical from that Dish receiver.