I think this mast isolation idea originated from the J-Pole. The J-Pole is a unique design that inherently wants to radiate off it's coax and mast. Amateurs have long written about the use of coax baluns and insulating from the mast with the J-Pole. With this in mind I feel it really depends on the type of vertical you use as to if insulating from the mast will help or not.
From a design standpoint, most end fed fiberglass verticals already have a balun installed as their matching network. This device serves two purposes. First it matches the impedance. Typically from few thousand ohms back down to 50 ohms. Second, it gives us the benefits of a balun and reduces common mode currents on the coax feedpoint ground.
This is also the mast ground and the location you would connect the optional ground plane kit. If the antenna has been specifically designed to operate without a ground plane, then there is going to be little current available here to see much benefit in the ground plane kit or mast isolation. This is not to say that it can't help at all and that some won't see a benefit. Because the internal balun will never remove 100% of common mode currents.
In cases where your mast is resonant at the operating frequency, you probably can see a reduction in RFI with mast isolation. In most cases with some care taken in the installation you can avoid having to insulate the antenna from the mast on most verticals except perhaps the J-Pole. If your antenna does not use 1/4 wave radials, a coax balun can only help with RFI problems.