High quality cables are only really needed for analog signals and that is stretching it.
It depends on what kind of analog signal you want to transfer. Video can be picky and high-frequency analog signals require all kinds of interesting cable modifications to still offer acceptable signal quality.
Audio, on the other hand, is almost DC in terms of frequency. CDs use sampling rates of 44100 Hz, so according to Nyquist's and Shannon's theorem, you'll be able to encode signals up to 22.5 kHz. At 22.5 kHz, most high-frequency effects are negligible.
If a cable is so poor it can't even transfer digital signals, 1s and 0s, then that is one mighty cheap low low quality cable.
Digital signals, on the other hand, require much more bandwidth to transmit than analoge signals, especially if you're sending them over one wire serially. Most cables are spec'd to include some headroom, but you'll occasionally find ones that don't. This doesn't mean digital cables need to be stupidly expensive; depending on technology used, the protocol allows for error correction, so a cable has to be quite bad to actually produce errors, but: Analog audio over several meters of coat hangers: No problem. Digital audio over the same coat hanger wire: Could be problematic.
Digital cabling does require some thought during design and specification, so as to avoid getting undesired effects, but one properly made digital cable is almost always as well as the other.
I don't see how any cable made of copper can degrade digital signals to the point where the signal is corrupted and unrecognizable to the gear receiving the signal.
Oh, that's quite easy. Parallel wires can induce currents in each other, thus messing up the signal. Capacitance can "mush" the signal, rendering it worse and worse; external interference can completely destroy the signal depending on transmission method used etc.
Most modern systems use Low Voltage Differential Signalling for transmission, which is quite robust concerning external interference; when used for short distances (e.g. SATA), only basic encoding schemes (i.e. 8b/10b) are employed (which can detect, but not correct errors; but this does save valuable bandwidth--transmission errors shouldn't occur, so if one occurs, it's better to just retransmit the packet than to use encoding schemes allowing for error correction); when used over long distances, where errors are to be expected, fancier encoding schemes are usually employed sacrificing bandwidth for the ability to correct data on-the-fly.
All things being equal (which they almost never are) you need a lot more bandwidth to carry data at 3Gb/s than you do 100Mb/s and with a thin cable like they spec'd for SATA II there are going to be some definite distance limitations using copper.
With digital high-frequency signals produced by current equipment, it's not wire resistance that messes up the signal (voltages are quite low to begin with and through use of clever techniques like LVDS, they don't even need to be high), but other properties. Putting thicker wires in a SATA cable won't help achieve a working ten meter long version
I always look at the gauge of the cable before I buy. Monoprice is clear about what you're buying, often Monster is not.
Unless I'm pulling it through the wall or my wife will freak I do get the biggest possible. I also do like to see good (full) shielding on cables when I'm splicing them myself. Canare for example is nice wire for DIY.
I have one monoprice 50 foot DVI cable that is so thick (like a US Quarter in diameter) I felt like I was laying transatlantic telegraph wire when I ran it from my basement wiring closed to my Living Room media pc. Works flawlessly.
Gauge is not everything. Digital cables usually don't need to be very thick, correct twisting and shielding is much more important. You can't achieve correct shielding without having a somewhat thicker cable, however. Thick cables wont guarantee good signal quality, but very thin and flexible cables sold for cheap will almost be safe to fail. (That's why thin and flexible cables are so horribly expensive--they require much better materials and techniques to produce to still deliver the required bandwidth)
-huha
