The contention that Instructors are TESTED every two years by dint of an instructional flight is overstating the nature of the flight with an instructor. It isn't a test remember. I'll wager that at more than a few clubs, two instructors from the same club will go tootling off for a jolly and there won't be an element of testing. It is a formality, or possibly will be in the minds of the individuals concerned. The system we have in place now ensures that no instructor need be tested after their initial flight test and that is wrong. By introducing the mandatory test you are introducing an independant eye and a standard to the revalidation of the rating and you are ensuring that specific skills are being assessed.
You may find it preposterous that high standards, such as applied by yourself, aren't universal; and I'm not suggesting that a majority of FI's are so irresponsible, but come on guys, you have as much of a vested interest too.
As a postscript....any test to renew any rating also counts as an instructed flight. Therefore an airline pilots LPC/OPC counts? What relevance does that have to flying and instructing in light aircraft?