There was a very good article by LJK Setright a few years back, about the ever more pervasive idea that smooth=fast.
One important point is that even if you did absolutely have to be smooth to be fast, this does not mean that a smooth rider is always a fast one, You can also be smooth and slow, so you cannot say smooth=fast. look at McCoy a few years back.
His main point, though, was that if a rider or driver is all over the place, then getting them to smooth it out may be good for getting them to speed up, as it reduced the loads on the tyres. There cold be occasions, though, when you wanted the car to be unsettled. As techniques become more advanced (and I am not a good enough rider or driver that this matters to me), you cannot rely on the fact that smooth is always fastest.
If, for example, you plot lateral and longitudinal acceleration on a graph for a car, the limit of grip will be something like a circle. Max sideways acceleration leaves no braking available, for example. If you are at one point on this circle, though, and want to be on another, there is an open question as to whether you shoudl transition smoothly around the perimeter, or move directly across the interior. This will tend to be less smooth, but may result in a faster lap.
