How useful would a 100 mile road be if in the middle there were a 10.6 foot-wide by thirty-foot deep ditch? A road which is 99.998% usable should be just about as useful as one that's 100% usable, right?
A device which can handle 90% of the tasks a car can do, but which can't handle 10% of the tasks that require a car, doesn't seem very useful unless it can do many of the 90% of tasks much better than a car. The Segway just plain isn't there.