There is a little effect when it happens, that's the best bit. But to have this as a gold talent is odd when there are already earlier talents that do 6% extra for Templar's Verdict and 15% extra for Divine Storm. This is just those, again. It should have an additional modifier, like "Sometimes those abilities cost no Holy Power"
And yeah, Ashes to Ashes is the gold to go for first, but the path to get to is so shit that all the guides tell you to get there and then do nothing else, saving up 67,000 Artifact Power so you can do a reset and get to it via a better longer path.
Yeah, ret feels weird, even for a 'support' DPS theme (which, incidentally, is not something I imagine that many, if any, DPS wants to be told)
The divine storm fling golden talent just feels like it's going to introduce a lot of hassle in trash pulls, and while I'll have to see the math when we get bonus traits for, say, Righteous blade with 2 or 3 bonus points, it just feels weird to imagine using DS on a single target rotation.
Plus, as you say, we have other tree points for BOTH Templar's verdict and Divine storm damage increases, so just having Ashbringer duplicate 10% of 40-60% (just going off of my logs, which admittedly could be off) of our damage isn't even a flat 10% increase to damage as-is.
There's some moderately cool stuff, like the one tree point which makes Divine storm proc friendly auto-heals, but....still super odd.