The signal running over HDMI is not packet-based. You don't have any mechanism like with TCP where packets can get dropped and resent later. So if interference causes bits to flip on an HDMI connection, you will immediately see the interference on your TV, and it won't be pretty; it would probably look something like an HDTV antenna signal that loses reception--missing or discolored chunks of the screen or sparkly pixels. The effect is immediate and disruptive.
The end result is that Monster Cable might do a better job of keeping your picture intact during a freak electrical storm, but there is no practical difference otherwise. If you are able to see a picture with HDMI, and you don't expect any wildly varying levels of RF interference in your environment, your cable is as good as anything else.
It's just a lot less likely.
A $10 cable wont fail 25 times within the lifespan of the technology it's attached to. HDMI won't exist before you get 25 failures, unless you enjoy hammering nails into you cables or some other bizarre behaviour.
One has to wonder if Monster and their retail partners would make just as much money selling them at $50, due to higher volume, with the added bonus of people not totally misleading everyone with the value proposition.
You are right, people will pay more for better quality but misleading people into believing something is better quality by outright lies actually violates most consumer protection laws. I've seen no $250 monster cables at best buy here in ontario, the most expensive I saw was a $60 10ft cable by monster, however the consumer protection act is very strong and very strict here. Best Buy has screwed me over before and the mention of the CPA to a manager gets the problem resolved quickly. It appears they know they're in the wrong in many cases, but people let them get away with it - this is usually on their extended warranties from my experience.
I'm not sure why you brought ethernet into the mix, but generally you're talking about much longer runs than HDMI and it's often not as easy to see when your are losing packets without specifically checking if you are dropping packets (IMHO poor network performance IS failing outright, but not as easily seen as a bunch of blocks on the TV). Even then, the cable you use is either delivering all the packets or it isn't (up to an acceptable level of errors/loss for the length you are running). Delivering a stronger 1/0 isn't going to change anything as long as it's strong enough for the receiver to differentiate. That's where it either works or it doesn't comes from.
Presumably on ethernet you have to decide if you want the Tx or Rx signal to have the extra digital nuance?
Really? Crap. I've just run the cable down to a back room (for an x360) - and it goes straight alongside a couple of flouro tubes.
There is a reason why every single TIA/EIA 568 cable plant is carefully tested for things like Near End Cross Talk (NEXT), Latency, Skew, etc..
There is also a reason why there is an entire _discipline_ dedicated to ensuring that the "Digital" signal not only has a channel, but a meticulously crafted one. (See: http://www.linktionary.com/t/tia_cabling.html for details)
To reiterates the OP - there is no "Digital Signal" - signals are analog in nature, and they need an analog channel to transmit them. That analog channel needs to be engineered carefully to get the lowest EBR (Error Bit Rate) possible.
What's interesting is the Failure case of the Digital CODEC on the other end - some may reduce transmit rate in the presence of an increased EBR resulting from dropped packets. Or some may seize up and cease to function. In the degraded case, having a pristine analog channel from the transmitter to the receiver will certainly result in a better digital signal.
There is also the issue of future proofing - We're installing an entire plant of Category 6a cable right now in one of our data centers for a gigabit environment - Category 6a is like the "Monster" version of Category 5e/6 - It seems like a douchey way to do a network install, except when you consider the following - (1) this cable plant will be in use for 10-15 years, (2) We're testing a bunch of Nexus 5010s that will likely make good use of that 6a for 10 Gigabit. So, in addition to passing _today's_ signal flawlessly, it will also pass tomorrow's. This might be an important factor if your HDMI cable is being run through hard to access cabinets/walls.
Now, with all that said, it is still probably the case that the $150 Monster Cables (under normal circumstances) don't have a lower EBR than a $10 cable, but, in the face of RF interference, It may actually be a better performer. I don't know, but I wouldn't discard the possibility without a bit of research and education.