hmm, that's actually a good idea, I dont think its anywhere near whats needed overall for tamers, but if you counted as 1 slot in animal form, youd either have to leave the Bake/Mare in the stable, or be on foot stealthed(slow) or be mounted and visable.
Good Idea in my opinion.
Ps: for you Special tamers that would mean leaving that Red-Headed Stepchild Frost Spider in the Stable for more schooling....