Hierophant: using Arcane Surge support or not?

On my current Hierophant I got Acane Blessing, which gives Arcane Surge when I hit an enemy with a spell. Path of Building is telling me to use Arcane Surge support in my 6-link eventhought I have Arcane Surge.

Is Path of Building wrong, or that makes Arcane Surge pretty much mendatory anyways?

You gain AS when you or totems hit an enemy with a spell.
You do not need the gem in your equipment.

If he can regularly proc it it should be very good no?

The gem will give an additional 10% cast speed and an additional 25% more spell damage.

You get Arcane Surge from your ascendancy, but that's only the basic version. Arcane Surge support scales with gem level and adds a more damage multiplier for the supported skill if you have arcane surge. It can make a big difference a higher levels. In short, arcane surge from the support is stronger than the one from Hierophant and thats why it may be recommending it to you.