While digital is digital, that doesn't mean you get 100% signal or nothing at all. Maybe 80% of the bits are correct, or 90%, or 99%. When you start approaching the point at which some bits are being dropped, the signal becomes unreliable and glitchy, and not necessarily on a consistent basis. So a poor DVI cable could easily cause glitchy, splotchy video.
But you would probably notice it and throw that cable away.
Luckily the newer cabling systems (DVI and 1394) are pretty well speced out, so that any cable that complies with the fairly rigid specs will probably work. But note that DVI especially is a very high bandwidth, fragile signal. The spec only allows cables to be 5 meters long (or something like that) before the signals get less reliable.
I personally own some of monster's lowest end cables, the $12 RCA interconnects available from Target, and I think they are decent. I totally agree that most of their cables are (like other audiophile cables) grossly overpriced. Knowledgeable people who know what is important in a cable can get good cables for much less. But Monster is effectively serving the large market of people who don't have be time or interest in understanding what makes a cable good, but want a trouble free, decently performing cable.
Anyway, as Ray said, there are many reasons for wanting good cabling, from ease of mechanical connection to reliability to cable flexibility to fashion. I am fine with Monster serving the market of people who want more and are willing to overpay a bit.
-Tom
