you can check the tradeoff between increased mana and increased damage after applying the point, before you confirm it by clicking the checkmark.
The short answer is, as you spend more points per element, you get a higher damage per unit mana increase. It makes a lot more sense to put all your skill points into one element. I have six points in air, and my spell does around 70-100 per shot at a cost of around 90 mana.
Putting one point in each will result in a much lower damage at a much higher mana cost.
But, if you encounter a monster that has immunities to one element, you'll be better off with the points spread. Personally, I don't think it's worth it, but that's the tradeoff.