Optical is immune to RF interference, but most quality cables are shielded enough for that to be a non-factor. Not sure where resistance came from.
HDMI enables the lossless audio from blu-ray discs to be used, which as Kal said can't be used with optical. For DVD playback, HDMI and optical will be the same using legacy Dolby Digital and DTS 5.1 lossy codecs. But HDMI carries audio and video, so there is usually no need to use an optical cable with whatever playback device you're using.
Don't believe everything on the Interwebz! A duck's quack DOES echo!