So far I haven't seen anyone question why the dedicated circuit made such a difference. So here is the answer: typical wiring for receptacles is several, often two or more rooms worth, on the same branch circuit. Aside from the smaller consequence of the longer cable run to reach the last receptacle on the branch circuit, the major consequence is the number of splices or receptacles that are in the path. This is a major consequence because the splices are often less than optimal, and where the branch circuit is wired through one receptacle, and on to other(s) it is usually with the backstab connections. A chain of many weak links is what you get, unless you properly pigtail wire each receptacle.
With that myth debunked, let's look at another myth: isolated grounds in dedicated branch circuits. By definition, when you do a dedicated branch circuit run, the ground is isolated. So what do isolated ground receptacles exist for? The answer is in commercial branch circuits, the circuit has a common junction box that each receptacle is wired into, in a hub and spoke topology. This can cause undesirable effects to the shared ground, from eddy currents induced in each spoke. So, where we want a "clean" ground, devoid of such electrical noise, we run a dedicated ground, and use an isolated ground receptacle to keep the noisy branch ground separated separate from the equipment safety ground.
