quote from Team Radiant (responding to additional units like the Magma Smith):
…our current take on magic is that magic will be rare in the game … so you can imagine a wizard type unit being an epic type unit … we almost think of magic like a nuclear power, you can do a lot of fantastic stuff with it, but the more you invest in it the more chance you have of having a disaster.
and a host of threads on the topic, that might be of interest (nothing that seems merge worthy here though):