With the transmission of any electrical signal there is a thing called loss - put in basic terms this is either through the poor resistance of the cable or it leaks out through its shielding. What you also have to remember is - if it can leak out - then other things can get in.
I agree that digital is either on or off (it works or it doesn't) however there is still a degree of not quite there and all there - this is normally presented in the digital domain as pixillation on pictures or sound drop out in audio.
Sky will probably provide cable which is up to the job - but it will definately not be the best by far!! Each installation is different and lots of things can affect the end signal ie length of run, other equipment, other cables, transformers (a bugger for causing Radio Frequency Interference) and so on. So I am firmly in the camp of get the better stuff - it will serve you well. It will have better screening, be less prone to RFI, use better quality copper and should hopefully keep the weather out.
Thats my ten penneth worth - pobably a load of tosh - but that is why I buy good quality connects.
