No, I'm not a salesman. Just reporting on my first-hand experiences.
I too once thought that the concept of "high-end" cables actually "sounding better" was false. But after I took the time to listen for myself, I learned that it is true. Again, I'm talking about unbalanced instrument cables here.
If you really feel so strongly against my claim, why don't you take a listen for yourself before you make accusations? Have you conducted a shoot-out of your own?
As others have stated above, differences CAN be heard. Subtle, but detectable. Again, it's a personal decision as to whether or not the degree of subtlety makes it worthwhile. But if you have good monitoring and good hearing, the differences are indeed obvious.
Yes, you'll detect differences in frequency response, but also in dynamics. Cheaper cables have more of a "saturated, compressed" type of quality to them... and you might actually prefer that. I happen to enjoy pure, authentic dynamics. If I wish to saturate / compress, I'll do that with outboard gear.
Also, a high-end cable will yield, what I call, a "higher-resolution" signal. The signal sounds more detailed and refined, extremely accurate and "present". By comparison, a "lesser" cable will yield a slightly more "blurry" or "cloudy" signal, darker, dirtier. None of the "good, expensive" cables I tested were blurry or cloudy, but a few standard ones were, even ones of known, respected brand names.
Now, if you are going to feed your electric guitar signal into a super dirty amp with tons of overdrive, multiple effects pedals, through a super saturating, distorting mic pre-amp etc, then likely you do not need the pristine fidelity that a Mogami Platinum cable provides. After all that processing, the subtle refinement the good cable offers will be completely lost. But in some cases, depending on what you're recording and how you're treating it, etc, the extra fidelity of the good cable surely can be enjoyed.
Again, it's all a situational and personal decision. If you choose to not believe it without even hearing for yourself, that's also a personal decision and is fine. Believe whatever you like. But in that case, at least refrain from posting false accusations.