I had an in-game discussion about this quite awhile ago, but only got one person's view.

There are items a thief can use in the Tree, but to my knowledge, I've never seen a thief in the Outlanders. When it was discussed, the general consensus was that thieves are tied too closely to civilization and are not to be trusted or traveled with. Were the items for the exception to the rule, or were they intended to be used as a normal thing? Also, are certain kinds of thiefs disallowed within the Outlanders? I could understand that any thief who joined a guild in a city would not be admitted. Also, its well known that assassins lose hide and assassination. Do thieves lose anything other than hide?

I don't plan on creating an Outlander thief in the near future because it would make me really obvious. But then, perhaps if there were some information about the view of the Ancients to this matter, there would be an occasional player other than me who would be willing to give it a try.