Not all people live where they can hire a painting contractor, like you describe. People who live in small towns can only hire painters who have a very small business, and do two or three paint jobs per week. In this case, you do have to be very careful, when you hire a painter, as we have several, in our area, who are out to make a fast buck anyway they can.
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.