Hi Mason,
I use \shapeII extensively.
I think the point about engraving is that while rules based, it is an art, and the result depends on the skill and good taste of the user. There is no definitive 'good slur'. The function therefore has two uses, one to fix lilypond slurs that are not entirely felicitous, and the other to shape to your taste.
As for predicting the result, you get to know when you use it as much as I do - just experience. I myself always use polar coordinates as to my mind the angle indicates the trajectory clearly. But the function is very flexible.
Just my two bits worth.
Andrew