What can I do with poison potions? I don't see any point in poison myself. Can I combine it with weapons or use it on enemies?
No, you can't combine the poison with stuff at the moment, but it might be a planned feature.
But you can create a splash potion out of your poison potion. Because you can throw a splash potion it can be used against other entities. Just beware, skeletons and zombies don't take damage from them. To kill these you need a splash health potion. Source
As that wiki page says, undead mobs and spiders are immune to poison splash potions.
You cannot currently use poison with weapons.