This is a subject that could be debated all day. The reason for signing is to identify that the required flight has been conducted with an instructor with their knowledge. As an examiner you have no real way of knowing who is and isn't an instructor. At least if the flight is signed for, you can check with the CAA if in doubt.
The purpose of the flight is to ensure that every pilot flies with an instructor at least once every two years. The guidance on what that flight should contain was produced by the CAA because instructors at Seminars requested it.
At the end of the day one flight with an instructor may be meaningless but, for the many PPLs who have not flown with an instructor in the past 20 years, a little purposeful training may go some way to preventing accidents.
If an instructor does not sign, it does not make it a test in the licensing sense, you can still fly whilst you licence remains valid, assuming you have an aeroplane. Maybe a second flight with another instructor will confirm your ability or lack of it.
If you hire an aircraft you get a check out, if you perform badly, you don't get to fly solo, thats at Test! and instructors have been doing them for years.