On reason for joining the goblins: There is a goblin prisoner who tells you that the goblins' priestess can help you with your tadpole.
From what i've seen you don't actually have to join the goblins for that. You can just go to their camp and talk to the priestess, and considering she doesn't cure you or offer any tangible help really.. at that point you honestly are stuck with 'no reason to join goblins'.
