This card is extremely strong with cards like Eye of the Dreamer or Breath of the Sleeper .
When you have Spirit-Speaker and one of those spell cards in your starting hand, you can have 10 ressources (5 ressources net) after two actions.
10 Ressources - 5 Ressources - 2 Actions - 2 Cards = 1 still doesn't sound worth it. Well Spirit-Breaker isn't lost so it will shine when you use it again, no matter if for ressources of for returning an asset.
A lot of other cards transform into optional emergency caches or better, only needing the action when you have to play them first.
There are some cards (especially Rogue cards) that do better, but they are just an instant gain while Spirit-Speaker can be used whenever you need ressources AND it still has another use which some players have as primary use (returning an asset to your hand, especially high XP cards).
Quite often there are those spells you only use situational, like evade spells. When you feel you are better equipped to kill an enemy than to evade it, you can transfer the rest of the charges into ressources, so you have effectively cleared a slot.