The passive skill Perfectionist reduces the discipline cost of all skills by 10%
That means we will save a whopping 0.3 - 1.4 discipline depending on which skill is used. Or 4.5 for preperation, which has the highest discipline cost (and only 2.5 if you are using the punishment rune). I think this will make very little difference and would almost be a waste of a skill slot. I think they should increase the discipline savings to 20% to make this passive worth choosing over any other passive skill.
They sure did some major changes in the last patch. I was really pumped about DH, but they practically stripped away all the synergies any build could have. Now it's too straight forward for my selective taste.
keep an open mind, there are a lot of builds you could use, maybe it's easier for me to see since I've only extensively looked over the demon hunter, as opposed to looking through every class. personally I plan to try and chain as many skills together to make a glass cannon like build with few defensive skills.
on topic, Perfectionist is an decent passive, It could use a bit of a buff for sure, but I think it would shine in a build that used heavy discipline or had discipline skills you planned to spam.
Even though it's the first passive I think it will come into it's own later in the game when you are sporting a higher level build with possibly increased discipline pool size and better hatred generation etc.