If a godclad becomes stronger and gains more spiritual power, the boost from the mastery of the skill will only increase exponentially and it will be free. This is why gaining mastery of an ability is very important.
Increasing spiritual energy is simple and will cost money. But increasing the mastery of skill will only cost time and effort.
In the case where they are already spending money to buy energy water in order to increase their spiritual power, it will be prudent to spend the extra spiritual energy gained from energy water to train ability mastery too.
After all, the gain in spiritual energy from energy water is temporary. They will be wasting it if they don't use the temporary spiritual energy for nothing.
They don't like wastage normally. But when they have a way to increase the mastery of their divine abilities quickly, they have to practice their abilities with the extra spiritual energy.
