Well I wouldn't say they are full of shit. I'll post an article about this as soon as I can find it, but over feeding your plants can have traumatic results on your yields and flavor! Mineral salts are bounded, and let's say your plants are in desperate need of Phosphorus, but are over loaded with Sodium, well your source of P happens to be TSP (old school I know, it was the first example that came to mind) so your plants cannot take up any more Phosphorus without taking up more Sodium. Well because we bumped up the PPM's so high we now not only have plenty of Sodium, but a Sodium toxicity. This means the plant will have a hard time getting rid of this build up, so if it is close to crop we will have crackling and harsh buds. Not only this, but plants with a nutrient toxicity lose their resistance to disease like root rot, bud rot, and powdery mildew. Like I said, 100ppm's has been the difference between a crop of 6 week plants finishing up, or a crop of plants infested with root rot.
There was more than just res temps that screwed my friend over with mold, he also ran his PPM's well into the 2000's, and because of this he could not control is pH. When you cannot control your pH certain nutrients separate from their original chemical forms and turn into a cloudy solution your plants cannot take up, this begins different toxicities which start deficiencies else where, in the end your plants get disease and bug attacks.