Picture the scene, your Seminal or Senepod comes out of it's annual greasing. You don't have to do a flight test any more, the aeroplane is used for twin training. Next time out it loses one for real on climbout and you end up in a field because nobody tested the single engine perf after the annual check.
Boy are the lawyers going to get rich !
I have just had the new EASA C of A done on my Citabria. I then went up and did a flight test for personal peace of mind.
Glad I did, a couple of minor problems showed, easily remedied but may have gone un noticed without a specific test.