actually, cable quality are important for both analog and digital. although for digital, the "requirement" are not as strict as analog, but still very important.
example:
for Ethernet cable: CAT5, CAT5e, and CAT6 are all 8 cord twisted cable. but CAT5 are just not as fast as true CAT6 cable.
for USB cable: i have 6 USB cable from cheap no brand to belkin USB2 certified cable. when transfer small data (smaller than 1GB) all cable works just fine. However, when i do disk to disk copy with external 2.5 USB when USB cable will transfer both data and power for the HD, only the belkin works.
i am not saying someone should spend hundreds on cables. however, they do make a difference.