Chuck,
I would disagree. I think that more than 10% were better quality rectifiers and it was probably closer to 40% or more. Many of the brands we drink today had their start as rectified whiskey including IW Harper, Four Roses, Old Forester and even more recently, Wild Turkey. Good quality rectifiers were common enough that they were able to prevent the "Imitation whiskey" from being applied to their products. If 90% of them were making a cheap product that contained unknown elements, I think they would have been forced to accept that label. I think Taft made his decision in a way that protected that group of rectifiers making a good product from being called "Imitation whiskey". I don't think he would have done that if they were only a small minority. It was the straight whiskey propaganda that makes people think that the majority of the rectifiers sold snake oil.