Not only do battles combine a great element of strategy, with the synergy you can develop between pets some being more buff/defense oriented and the others attack. Of course then once you find good combos you have to level them up... and find a good complimentary third to take care of enemies you may be weak against.
but you also force people to think about the composition and attacks of their team, making sure not only will you have attacks of enough different types that they won't be week against everything your opponent has, but your own guys aren't all going to be slaughtered by him because he does extra damage to you.
What am I saying? I am saying I have spent way too long having fun hunting pets, leveling pets, battling pets to actually go out and level my own toon. Can you please nerf the amount of fun Pet battles are? Otherwise I will never hit 90.