The reason? Well for one, i'm pretty sure I read somewhere that his future expansion will be against the Sorceror, who is arcane/dark trained. Arcane/Dark vs Nature/Mind is more thematic than Arcane/Dark vs just nature or nature/elements. Besides, we already have a mage trained in nature and level one of an element. Futhermore, it's been said that the Shaman will use totems. Have you seen the totems in the core set? Tooth and nail is just a statue of a lion with his clawed paw taking a swipe. This statue gives all animals in the arena piercing +1, just like the lion it depicts. Rajan's fury is a statue of a stampede, and gives all animals in the arena the charge +1 ability, so they can stampede too. And animal kinship has the likenesses of five different beasts carved into it, which gives its controlling mage their powers when the same creatures depicted in the statue are summoned.
In other words, the shaman will be skilled with sympathetic magic.
I notice of course that the totems in the core set aren't nature/mind spells, only nature. I bet he also has an ability to copy an enemy's ability or somehow use their powers. The reason I think this is that the totems that currently exist are focused on buffing animals, but we already have an animal mage. If the shaman uses animals, it will be partly because he is trained in nature, and partly because the beastmaster uses animals. Therefore, I suspect there will be totems that synergize with both the shaman and other mages, just like the totems that the beastmaster likes to to use. Either that or the shaman really specializes in animals, which I find unlikely.
I know it's a long way off, but it's never too soon to start speculating, especially since it might be highly relevant in the fanfic I'm writing of Mage Wars: Nature of the Beast. Thoughts?