I like that mana is a resource which has to be managed.
And that you have more then one tool for a given heal.

In today's game, you go for the biggest heal possible, as often as possible.
And because of the mana pool size and mana regeneration, the costs are not much of an issue.



But as an analogy...
You have 100 mana, for this entire fight.
Your small heal costs 5 mana and heals for 5.
Your medium heal costs 15 mana and heals for 25.
Your big heal costs 25 mana and heals for 50.

You cannot just spam the big heal on everyone; you'll run out of mana.
The DPS will almost never require heals for 50, and its a quarter of your mana.
It is the most efficient heal for the tank, provided the tank will survive for it to land.