I have noticed this tendency for some pilots to be very active on the controls - particularly during final approach and through flare. I am very curious what others who have noted this would have to say regarding the frequency range for such inputs. Particularly at low speeds (such as approach), transport category aircraft have response bandwidths that are half a Hz or less. Inputs at frequencies higher than this (less than two seconds per cycle) will have little impact on the airplane's attitudes (pitch or roll) and even less impact on flight path. One theory I have for this is that some pilots feel that they want greater bandwidth out of the airplane response and thus tend to drive the controls aggressively at higher frequencies thinking it will help.
A few entries back a question was posed as to whether or not this stirring the mayo actually causes the surfaces to move. The answer is yes. Transport category airplane control systems that I am familiar with have surface rate capabilities on the order of 50 degrees or more per second. For a control surface that has a stop to stop travel range on the order of 50 degrees, sawing the pilot controls back and forth half travel at 1 Hz will drive the corresponding surface(s) at their rate limits through a range of 25 or more degrees.
Abrupt inputs at higher frequencies will tend to stir up flexible structural modes. For larger transports the associated modal natural frequencies can be as slow as 2 Hz or less. Feeding energy into body flex modes does nothing toward controlling airplane attitude or path, but sure degrades the ride quality. One name for this is pilot induced turbulence as mentioned earlier in this thread! I wonder if pilots with experience on larger airplanes that tend to exhibit more flex effects have learned to resist being aggressive on the controls because of the negative impact of driving the flex modes while pilots on smaller airplanes don't get as much feedback from the seat of the pants that high frequency inputs are not a good idea.
FBW airplanes with stability augmentation control systems use both pilot controller inputs and stability enhancing feedback signals to command the control surfaces. If the pilot is really stirring the pot with large, higher frequency inputs the result can be that the surfaces spend most of their time sawing back and forth at their rate limits. When the surfaces are rate limited due to pilot input they are not able to simultaneously respond to stability augmentation feedback commands. The augmentation is essentially lost - not a good situation for a relaxed stability configuration.