The Spirit Mage role in the deck is to be the cheap unit.
Often you want to attack with a unit from a position which would leave it open to counterattacks in a bad way. Abua Shi’s units are expensive and have high life, not the kind of unit you want to freely leave around with exposed flanks waiting to be picked off. This is especially true against some high attack champion that your opponent may have summoned for the express purpose of dealing with these tough Savanna Elves units. Spirit Mages are cheap and do most of their work once they are summoned, attacking with a ranged strength of three and granting a boost to a unit, so don’t be afraid to leave them exposed. If you have no immediate use for his boost, you can always store it on Abua Shi for later use.