Since the intended purpose of the Pet Balls was to help players get stuck pets, I don't really think it's fair to make it only available to those with the template space for another skill. Crafters should also be able to access their packies/beetles should they become stuck, or inaccessible. I think Herding definately needs a rework, no doubt, but I think it shouldn't replace Pet Balls.
Maybe for those that use the Pet Balls as a "weapon", the Herding skill could be reworked to lessen the Reuse timer, should that be implemented. Or give a chance to stay hidden, etc.
I know I want more stable slots, too! But same with above, I think this should be equally accessible to all types of characters, without having to add another skill to the mix, especially since I don't see how more stable space would cause any imbalances.
Good thinking of the Herding skill, though...I would like to see it become useful in the future! And of course, this is just my opinions, others may think this is the best option, so thanks for sharing your idea. =)