RFI/EMI is noise. Noise is the plague of the audiophile world. It is what every hifi maker is fighting in design, and what every hifi consumer is fighting with better power cables, USB interfaces, NAA, etc. The Wave BNC cables and the optical "gap" solutions are all attempts at reducing noise. Noise affects the sound directly and also by its effects on the digital processing cycles (aka jitter).
By "noise," we don't mean audible hiss. There is always a level of parasitic noise that corrupts the pure signal which isn't as obvious. How do I know if it affects the sound then?
It's not as easy to detect as bass/treble response, soundstage imaging, or dynamics. You have to learn to listen for it because you're not trying to pick out the sound but how its being compromised. Why in the world would I want to learn to listen for something I don't want? If I'm not hearing it, then good right?
It's like there's, I don't know, a layer of dust on your TV screen. You didn't put it there. You can't see it. You can see the brightness, contrast, color, and resolution, and they're fine. However, when you take a microfiber cloth and wipe away the dust you go, "Oh, wait...."
You're listening for the way the sound is dull, grainy, glare-y, digital-ey, etc. The more resolving your system the more you can pick it out and the more it's worth rooting out. You may not hear it at fist but you can feel its fatigue. Then you analyze what exactly is fatiguing so you can fix it. It's not always noise, of course, but a system is always improved by dealing with noise.