Hmm... I would specify that they can form other manipulatory apendages, but that they are very short and weak... if only for fluff reasons. Trying to do much crafting with only one arm can be a bit troublesome... Also I assume they can't use a shield and weapon at the same time?
What about Mage Armor (including Bracers of Armor?)... they would make very poor wizards due to the intellegence penalty, but good Sorcerers and/or Monks (assuming they still would get a monks number of attacks and flurry).