Sanctuary of Thought - mana build

I was just wondering what was the reasoning behind Sanctuary of Thought -50% skills damage cost.

Mana build usually revolve around spending and recovering as much mp as possible.

20% of max mp makes for a quite decent layer of defense if you have 7-10k mp.
Then obviously the increased area of effect which kind of make perfect sens with it. Easy 100% more AoE.
Mp reservation, not sure why that is there but sure why not.

And then... -50% cost of skills...
Doesn't work well with stable mana gems such as archmage, arcane cloak, sigil of power and the one with the square aoe (for what it's worth).

I've checked on PoE Ninja and it's used a bit with hexblast but... most of them have actually low mp (like 1500-2000) and use eldritch battery.
And I'm just wondering why an ascendancy with so many "increased maximum mp" ends up being used as anything but mp.

Beside some quircky build or wanting to try something new/special, is there any reason to go Hierophant for manabuild? (with sanctuary of thought obviously)