I am not sure if this is intentional or not or even if it is exploitable really but the spell gives your animal companion DR as well as the +1ab and +1 magic damage. I read the spell description and it says nothing about giving DR only being able to bypas it. Perhaps it has something to do with the coding that allows magic fang to be casted on reptile domain summons?

Magic Fang does appear to grant DR 1/+1. According to the script, this is needed to grant a true enhancement bonus rather than just an attack/damage bonus. I don't know if this is a pre-HotU leftover or if the attack bonus-beats-DR bug only applies to weapons (Magic Fang is not applied to a weapon).

However, I can't find any reports of this "bug" anywhere, and it's not even listed in the wiki. While the spell technically is more powerful than it should be, it clearly hasn't been enough to ever unbalance the game, so I doubt it will now. Don't know if it's worth changing.

I don't think we're going to do anything about this. <.<