Tri-wheel robots wobble really badly. For telepresence, the screen is at the top of a mast so wobbles are amplified. The balancing robots have very smooth motion.
No, it's very easy and inexpensive to balance that robot at this point for Segway. They've already solved those problems, it's a natural extension to use their existing technology for stabilizing the robot. Further, it's not an option to not stabilize the whole robot, you have to or the whole concept implodes. Put another way, they were going down that road regardless.
Put it this way: The head on the end of the mast is a significant mass at the end of a long lever. To dynamically stabilize that, you're going to need big, hefty, expensive motors. Well, you already have big, hefty expensive motors to move the thing around in the first place, so it actually costs less money to dynamically stabilize the whole robot.
Check out the inverted pendulum on a cart problem, or go balance a couple bats on your hand. When near vertical the inertia of the pendulum mass helps you out, it's the inertia of the cart the motors need to overcome. The better the control loop the fewer and smaller the corrections will be, tending towards zero.
I have a video camera with optical image stabilization. It uses teeny, tiny cheap motors to move some optics around to stabilize the image. It is not obvious that stabilizing video justifies giving up the passive stability of a tripod.