I might be wrong but I believe Bob used it in all of his paintings on his shows, it's the wet on wet technique. If he didn't mention it, he probably just forgot. Anyone starting out I would say yes use it along with the advice not to use too much, using the technique the way Bob intended. I've started to use it less now, only in selective areas such as the sky and water where I know I am going to be blending, keeping the rest of the canvas dry. This way I don't make so much mud when adding layers. I've found that the smoother the canvas you use, the less slip you need overall, and in places where it's not needed I don't use it unless I'm using a rough canvas. This wouldn't suit everybody because paint consistency and canvas type come into the mix, so it's trial and error until you find your own right balance. Bob's been showing us a system that works but if you alter something, you might have to alter something else as well. I'm still working on it. Gamblin do a clear gel alternative which I intend trying some time.