Having worked for years in a trade paint shop, mixing several brands of paint for traders and retail customers, I'd steer clear of anyone who will only use Dulux.
Valspar paint is fine to work with but it does take a bit more finesse. You can't keep going over and over the same section once it starts to tack off. You have to get it right first time or wait till it dries and recoat.
Dulux is decent paint, nothing wrong with it but its the amateurs favourite. It is so thick and gelatinous that you can slap it on with a broom and still get a reasonable finish. Good stuff. Love Dulux but it isn't the only good paint.
A professional should be able to work with dulux, crown, valspar and anything in between. Only ones who insisted on Dulux were the ones who really couldn't be arsed and just wanted the job done as fast as possible with as little effort and attention to detail as possible.
Same as the ones who will only use matt paint, not silk. Silk is more durable, more cleanable and doesn't show scuffs. But you have to be a bit more meticulous with silk, keep roller strokes in the same direction and avoid splatters. Takes more skill to apply. You make a mistake it shows. Matt is easier as it doesn't reflect light, its dull so if you screw it up it hides it better.
Just my two pennies worth.