Question about cast on elemental ailment

I'm just curious why this cost more than , and is higher tier than elemental invocation? It seems like a complete downgrade. It goes off by itself, meaning many times it's wasted when going off with few or no npcs near. Where as Invocation can be triggered when needed. The only real downside I see is that invocation requires some dex and ailment is int only.

I just feel like I'm missing something here. Why pay 100 spirit for ailment? Do the skills not cost mana? I know invocation they do consume mana.

It auto casting is an upside, not a downside.

IIRC you can also charge it faster, especially with cold.

But hey, if you prefer invocation then bonus spirit for you ;)

BUMPing this post because I'm also curious how this interaction works and if there's any difference.