See this video: http://edge.technet.com/Media/Forefront-TMG-RTM-Overview-Interview/
In the linked video, it mentions that TMG is used as the "platform" of UAG, which means UAG is built on top of TMG. However, you cannot access the functionality of TMG as it is all handled "under the hood" by the UAG. Thus, you need a UAG for DA, since that is what it was specifically designed for. The other traditional remote access technologies in TMG are carried over from ISA in order to provide continuity with the product, but the separation of these products indicate that the long-term MS approach for handling "inbound" traffic will move to UAG exclusively while "outbound" will be handled by TMG.