This is why they put size restrictions on the replacement models in Counterstrike, back in the day.
The basic idea is, changing a model with another might give you an advantage in i.e. pvp. Kinda hard to hide with the flag, if the opponent has swapped the model of ur breed with one that has a HUGE FUCKING ARROW pointing from the head ^^
Same goes for changing the skins of things, to let it be easier to spot, be it ores, other players, special mobs, etc.
I don't know how or if blizzard can check if u have modified files or added files though. I know there was model replacements for druid shapes, and I tried one for travelshape a lil bit, and I've also seen some asians using the "tigron" models for their catshape, so.....