Miniature donkeys and burros will serve as protection if you have other livestock. They are very popular around here among those that raise small goat breeds which can be taken by packs of coyotes or dogs. A burro or a donkey will protect the rest of the pack quite agressively if necessary.

Of course, an alpaca will do the same thing...so there isn't much need for the mini burro/donkey if you've got an alpaca herd.

They could be used to pull small carts. They are cute and make good pets.

Actually, if you only want one alpaca or one goat, then a donkey/burro is a good companion animal as well. Most of these types of animals are herd animals and therefore need other herd animals around...even if they are not of the same breed.