You are not incorrect or correct. Experienced detectorists really like to move away from electronic discrimination when possible and "hear it all" - pure all metal mode is a form of this. They want their brain to do the discrimination and all of that noise does not sound like a cacophony of random noise but more like a symphony of discrete "instrumental signatures" that correspond to the various targets in the ground. You tend to paint a mental x-ray picture of what's in the ground using the sounds/tones. So awareness of ALL conductive targets in the ground via tones and thresholds is where experience typically leads folks. Furthermore, in some cases, use of discrimination filters can reduce the detection depth of even non-discriminated non-ferrous targets and so that is another reason not to use discrimination. Some cannot get there and also it can be something that is also mentally fatiguing to even experienced detectorists (their brain gives out before their body does during a detecting session) so you can't just listen to that forever on end. Threshold hum actually tends to lessen audio fatigue. It is like white noise. Some people actually need white noise generators (fans or artificial rain/surf sounds) to sleep.
You can always go silent if you want and lower the threshold to nil and just dig non-ferrous without regard to iron. But threshold is sort of like using iron volume on the Deus when discriminating out Iron. In fact, the Deus uses a threshold hum when using Gold Field mode which is a pitch based program this is closest to all metal that the Deus gets. It uses iron rejection rather than iron discrimination in that mode. Threshold just gives you the awareness of disc'd out ferrous so you can be aware that you are amongst iron. This awareness is important if you are searching for an old home site and there are no visual indicators only a preponderance of nails or other "trash" and being able to hear it lets you know you are there even if you don't plan on digging the trash. A lot of detectors use a threshold hum, even for non-ferrous. Sometimes, the only clue you may be over a "dig me" deep non-ferrous target is the threshold disappearing (because really deep non-ferrous beyond the ID range of the detector can be detected but it it can't ID it, it usually defaults to an iron tone/ID or just a loss of threshold hum). Running with a low barely audible threshold hum is pretty standard stuff with many detectors, especially PI detectors. HTH.
