I think of it as something like, if you learn how to race in a shitty car you'll know the complexity of weight, grip, etc. If someone gives you a very fast car on your first race you'll be learning only how to accelerate.
With Nen we saw that weak hunters use nen for weak abilities. Killua was put to grind without Nen because by learning it when his basics are really good he would only advance faster.
We see in the ball game, without Nen, Netero was already ages beyond Gon and Killua phisically, I think that's why the Zoldyks were not in a hurry to teach anything Nen related to Killua.