As someone who has not yet purchased any of the hero and monster packs, there actually has been numerical symmetry for the archetype representation, so far as symmetry. However, it can likely be argued that mages and healers are likely to be the least common types of adventurers in the world. That said, my gamer OCD doesn't like that, either. And I wasn't really trying to imply that FFG owed us anything.
I do see what people are saying about the lack of inspiration with Mage classes, but at the same time I feel like mages should have the most freedom, since magic can pretty much take whatever form you can imagine.
I just don't understand why they've expanded every other aspect of the game at this point, but haven't for whatever reason added these two classes. It baffles me.