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'.
You don't join the Goblins, you join the Absolute. When I got my mark, interesting things happened. I haven't explored it more, but it does seem there is something to he had here.
So far joining the Absolute seems to be the best way to learn the secrets of why you aren't turning, where as joining the Druids seems to be the best way to get rid of the tadpole.
I would like to see a better "pull to the dark side" BEFORE you get the first goblin fight, that way you aren't first met with anti-gobbo propaganda, with no pro-absolute propaganda.