Well, at the moment D3 engine means limited lighting (i.e, flashlight time) and small rooms and corridors, much more than that and framerates die. Having more flexibility would be nice.
They've got experience with T:V engine already so can probably do more with it, get things done quicker, etc. Also they could write new code into the engine if they wanted, it doesn't mean it will be limited to what you see in the Tribes game. Look at the difference between CoD:UO and Quake3, same engine, lots of new shit coded in.