Hi HazelNuts39,
What are the disadvantages?
We have that system already. With blocked pitots and your IAS shows say 60 kts, then the superimposed stall speed is uselessly displayed off scale.
control surface deflection:
The crew of A380 VH-OQA were concerned about their roll controllability with a big fuel imbalance. At a lecture I attended, the Captain mentioned that there was no feed back through the side stick as to how much aileron was being applied. They therefore frequently looked at their flight controls system page to see how close to aileron saturation they were getting. (only inboard ailerons were working)
see pages 6-7 of
http://www.atsb.gov.au/media/2888854...y%20report.pdf
"Prior to leaving the holding pattern, the crew discussed the controllability of the aircraft and conducted a number of manual handling checks at the holding speed. The crew decided that the aircraft remained controllable...As the crew started to reconfigure the aircraft for the approach by lowering flaps, they conducted further controllability checks at the approach speed and decided that the aircraft remained controllable..."
On Airbus, because the control surfaces are being moved "invisibly" (except on a systems page which may not be displayed) when the ailerons or elevator reach say half deflection, then I think there should be some alert. When the stab trim runs outside its normal range - then there should be a warning.