These dudes are quite useful. But even more useful in multiplayer. If you manage to get them inside a base they can unlock doors during raids. And they can throw poop! You can also equip them with hats which looks better on them than hoomans.
I had my monkey on my shoulder so it could fling poop at some creatures that I wanted it to kill. I had it target a turtle. After a while, the turtle started running away. About 2 minutes later, it falls to the ground, completely unconscious. So use a monkey for turtle taming, I guess.
I just tested this, but if you set them on wandering and throw them at a patch of bushes/bamboo shoots or whatever, they can actually gather a decent quantity of berries and seeds. Just be sure to clean their inventory so they can continue and then pick them up and throw them at a new patch of shrubs! Simple as that! Hopefully people find this useful