Special arrows are different than spells: First, they have no cooldowns, so when needed you can use a bunch at once. Second, they don't cost as much up-front as spellbooks, so it's easy to pick up a few in town as needed instead of having to save up for a while. Third, they can replicate the effects of several schools or magic or spells without requiring you to spend precious ability points.
The Twitch Q&A also did say that they'll be reducing the costs of the special arrows AND making the components to craft them a bit more common, and they also appear in loot.
