It certainly can make a difference, and that difference
might be quite visible.
You're right in saying it gets there or it doesn't but consider this:
1. The signals are error corrected. If all of your 0s and 1s arrive as they were sent, then the error correction software has nothing to do. If some have changed, then the error corrector has work to do. If a lot have changed, then it has a lot of work to do. HDMI is specified and designed to attempt to deliver the highest possible bandwidth and it's possible that the correction software can't keep up with high numbers of errors.
2. The quality you get is directly proportional to bandwidth. Cheap cables can put a cap on the maximum bandwidth possible. If you're viewing 720p or i with simple Dolby stereo, then you'd need a severe problem for it to become noticeable. However, it you're streaming 1020p with 7.1 channels of DTS, then you can get problems creeping in.
3. Length of the cable is a big factor and above 2 metres, then quality of the cable starts to impact the quality of the result. The average lounge setup won't see any difference but with a home theatre setup, where you may well have runs over 2m, the quality becomes very important.
4. Routing cable close to sources of interference like power supplies and so on can cause problems. I had jittering on my 1020p setup with Bluray. Routing the cable away from a power supply I have for powering a network switch fixed it. FYI, I use cables that IIRC cost £10-£15 and I don't see or hear any issues at all. I've got a couple of test discs that exercise sound and video and all passes OK - or at least, to my eye and ear ;-)
A good analogy is with trying to use a WiFi connection with a weak signal or lots of interference. The bandwidth can drop dramatically. Even though the signal is digital, it's sent over an analogue medium.
Check out this link. Includes a scenario where degredation of picture was very noticeable.
Bottom line, for the lounge with Bluray, HD TV and so on, all on a single rack with all runs less than 2m, no problem with £10 cables but for more complex setups, definitely spend what you can afford.
The Truth About Monster Cable - Cables - Gizmodo
Hope this helps..