Well, "elements matter" is a very reasonable way to play.
I just don't think it's the default.
Because it is not the default, you may have to change other game elements in order to balance things while using it.
For your games, I think it would be very reasonable for foci to benefit both offence and defense. Making elements important makes wizards weaker and foci less attractive. This change would counteract both, at least partially.
I think it'd work pretty well.
In fact, I can see a solid argument that playing your way with this houserule is better than "canon". (Scare quotes there to emphasize ambiguity and lameness of word.)