Some people choose not to use parroting, because they believe it will upset the tulpa -- and if the tulpa is sentient, it might. If the tulpa is not sentient ... well, it's a matter of debate. But yes, parroting is a very effective method, but you should stop once your tulpa can speak without it.
I wouldn't say that it stifles progress not to parrot. The way I see it, not parroting is the baseline, and parroting is a shortcut to progress.