I heard this a few weeks ago and initially thought the same as Genghis (in my very limited capacity as a pilot and engineering graduate). However, I have changed my mind.
Anecdotal evidence that I have heard suggestes that many aircraft that "fail" things like climb performance, get written down as the book figure anyway and my own personal experience supports this. Some of the flying club aircraft I have flown have demonstrated what I would consider to be fail points, such as an engine that always oversped because it "had the wrong prop" and a couple of other bits.
I also heard that the CAA have written down the climb performance of a light aircraft type due to the overwhelming test evidence - the reduction being of the order of 60 ft/min. Hardly a reason to carry out an airtest every three years.
If airtests were carried out "properly" then we may have a case, but I feel in most cases the aircraft is flown and figures manufactured to fit the paperwork (not critiscism of anyone here, just an observation of light aircraft star annual tests).
One question we need to ask, is what action has been taken due to airtests? If no significant action has been taken in all these years, then there is no point in doing them.