Buying a can of premium paint, then bait and switch over to low-line products? Again, complete nonsense. Think about it for a moment; the Painter needs 5 gallons of wall finish. So, he buys one can of premium and the rest 'cheap'? How is he going to hide the 4 other gallons? What's he going to hide it in??? He only has ONE gallon of premium. It's not as though he's going to keep older can labels, they would be covered in paint of a different colour.
Oh, where to begin? Let me start with 'watered down paint'. 25-50% before the material gets to the site? Impossible. You would basically be painting with water at that point. It would be less of a hassle, and cost, to simply use proper material. You would be forced to apply three coats instead of two, as the coverage would be horrible. Whatever cost you think might be saved in materials would be lost in labor.