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.
To be fair, where as the initial cost on arrowheads isn't as much as a skillbook, it doesn't take long for the cost to rapidly dwarf that of one. And while arrowheads don't cost ability points to use, they do cost ability points to craft. I'll point out that magic scrolls don't require ability points to use either and are also available as loot drops and through vendors.
You are correct on the lack of cooldown, although that is something that can be balanced through tuning the potency. For example, I would be surprised if an explosive arrowhead were as devastating as a casted fireball. And, again, magic scrolls don't have a cooldown either.
The difference is, scrolls are really meant for non-casters who find themselves needing a bit of magic. Mages don't need them. Where as arrowheads are for bow users; Rangers do need them (or would, if the game were harder - and it might be at later levels).
Hopefully the cost has been cut down dramatically. Perhaps arrowheads are intended as an "advanced" tool for the late game, at a point where the cost is negligible. But they're introduced from the start, so that's confusing.
Anyway, we'll know more in a few days!
