Couple of thoughts here. Again just thoughts not an attack on anyone or any particular paint brand! If all you ever do is use whatever paint the Ho buys or wants you to use, how does that separate your company from the rest of the pack? If you insist on using brands that you feel give the ho the best paint job, sooner or later it will distinguish you from the other painter's in your market. If all you do is use the Ho's preferred paint, all you become at that point is a price. That's the only way you can distinguish your company in the eyes of the Ho's. If you provide a better finish, a more durable finish, at a price that gives your customer the best value you become in their minds a little bit above all the other painters in your area.
Also, when a HO buys the paint, don't you think that kind of reflects on what the HO thinks about you as a painter? They're kind of saying in a way that anyone can put paint on a wall and the only reason they are hiring a painter in the first place is so they don't screw it up themselves. There is certainly no reason not to capitalize on people that think this way. You can certainly make money this way. But you will always be busting your butt to find that next job.
I have had painters that were quite successful doing it both ways. Some insist on using the paint that they want to use and some will use whatever the HO wants. But from my experience the painters that can afford to buy a new van every other year are usually the ones who use particular paints and walk away from the others. The main reason being the savings in labor they pocket as opposed to the painters who routinely struggle to apply crappy paint. The choice is yours, and there is certainly no wrong way to do it. But there is a better way, and that is to use premium paint products that give the best finish and are faster and easier to apply, and to sell that as part of your overall shtick.