I was trying out Demon Hunter on the PTR and Shattered Destiny is a cool talent, but it has anti-synergy with its prerequisites.
Shattered Destiny lets you stay longer in your demon form.
Both Restless Hunter and Inner Demon want you to weave in and out of your Demon form constantly to get the benefits.
It makes no sense to take Shattered Destiny if you pick either of these talents, as their boni are pretty much wasted if you managed to stay in your Demon Form almost constantly.
Do I understand something wrong, or is this talent shooting previous talents in the foot?
You got that right, but you also need to consider the fact that Shattered Destiny does not do too much for Demonic if you specced it.
It ends up maybe as 2, at very best 3 seconds of more Demon Form uptime, which means Restless Hunter and Inner Demon are still beneficent.
Also, you have CoH which allows you to get another Demon Form faster asap, which means more Restless Hunter and Inner Demon synergy, although the 10+ seconds added to Eye Beam on the Beta hurts if you dont play CoH.
Shattered Destiny finds big value for your 3 Minute Meta, but outside of that its just a little boost to get a smoother second Death Sweep into your haste-nerfed Demonic Window.