Is there some advantage later on to having a Ranger? Like is there another guild in brimloch Roon that they can join?

I think that the rangers are good for the party only for getting the vine magic skill, so thet you can cure diseases and poision without potions. However they cannot use the plate mail, so it is better not to have them in the endgame. To your second question- no, they have no special guild, they are just natural warriors and therefore warrior guild.

They're far from useless. Read the thread titled "Gold in the Gael Serran", among others that have been posted on the forum. Rangers are arrow manufacturers when they get the "Fletchery" trait (5th level Warrior's Guild). I wouldn't necessarily finish the game with one, but they're good as a "stepping-stone" role.

-Sazerac

Who needs plate armor? Rangers are definitely useful as a stepping stone one way or another, especially for that fletchery trait, and the whirling dervish spell works rather nicely on certain mobile trees. It's also a nice transition as you can wait until past Brimloch Roon to do it; unlike samurai, paladin, or ninja, you don't just have to be in one area of the map for the quest.