How important is EMI and RFI rejection?

Bingo. What Geoff said.

Then Goeff can get a measurement rig and show it. 

Oh, my! A one trick pony! 🐎

So EMI isn't measurable? The main source is AC mains noise and the harmonics. Using a frequency programmable power supply you can see this in action as you will see the fundamental change along with the spuria. 

Get an ADC and install ARTA will give you a host of spectrum analysis. Also there is the Dayton Omni Mic 2. 

Get a Schitt Modi 2 since it has basically no filtering on it's USB input and you can easily measure this. 

Again with the shoddy reading comprehension. Nobody said RFI/EMI can’t be measured. If you wish to pursue this line of illogical argument, why don’t YOU measure it and report back your findings? I’ll tell you why in advance. Because pseudo skeptics actually don’t get down and actually do anything. They just like to argue. Unfortunately their arguments are logical fallacies. I, on the other hand, am a real skeptic, not a pseudo one. I’ve taken the effort to measure RFI/EMI. People in the know go with the flow. 🏄🏻

"I don't know, Chief. He's very smart or very dumb." - Jaws