The reason for priming before texture,(rarely done these days),is that it allows the texture to adhere and dry evenly. When applied to a no primer surface, the texture reacts different on the mud joints vs. the paper surface of the wall board.This is crucial on knockdowns, because if you are inexperienced, you will be able to see all the mud joints after painting and different patterns of the knockdown, because the texture dries differently. When knocking down,the the texture over the mud joints will drag more than the papered surface. Another avenue instead of priming is to size the entire surface of the wall board with a light coat of mud,similar to a skim coat, only just wiping on and off. Feathering saves sanding.Not too many people i know do this, and as randymeyer stated, not doing it works for him,as long as the customer is happy with the outcome, that's all that matters. There is something to be said about longevity,however.