Digital transmission is either broken or it is not - i.e. if the signal gets through sufficiently to be detected 100% of the time, it doesnt matter if it was transmitted down strands of spider silk or solid gold ingots, it'll be reproduced the same at the other end. If your spider silk is SO bad that the highs and lows are indistinguishable the signal will come out as junk at the other end (and most systems counter this by ignoring the info altogether).
What SFB was saying is that as long as there is a noticable difference between the high and the low state and there are no stray spikes caused by noise sources, the quality of the cable matters not, the electronics will deal with it. A £5 cable with decent shielding will be fine. The signal will come out the same, it won't suffer degredation and you'll not know any difference. This isn't a case of "my directional cables are ace, you just can't hear the difference" - this is a measurable, scientifically establishable identicality in output. Analogue signals can be attenuated by poor cabling and suffer some odd changes, digital signals get those changes too, but the encoding of the data ensures that the content of the data is not changed by the quality of the signal.
A simple example is digital TV - watch your digital TVs signal strength bar on a channel - even on the ones that are right down to 25% the picture is identical to when it is 100%, it's only when it drops really really low that you get problems, and then you get corruption of the image.