Restricting instructor revalidation to individuals that you have actually flown the training flight with makes some sense. At least the instructor knows for sure that the training flight was legitimate. I have no idea how examiners varify the identity and qualification of an instructor that they do not personally know, and who could be based anywhere in EASA land. I personally would not want this responsibility.
Restricting instructor revalidation to immediately after the training flight makes it almost useless. The vast majority of revalidation candidates that I fly with try to get the training flight over with early in the 12 month period and do not yet have their 12 hours logged. I flew with someone this weekend who still needed an hour of P1 time before he can revalidate. He was planning to complete it this week, but now he will have to seek out an Examiner (who may not know me from Adam) to sign him off because apparently I cannot be trusted to have a record of my own flights or recognise my own signature in someone's log book.
Do we really want to encourage people to leave the instructional flight right to the end of their SEP rating validity?
What happens if the training flight was actually three flights?
If the purpose of this was to take some of the pressure off examiners, then it has missed the mark. I already have this "obligation" on my licence, but I suspect it will be rarely used and I wish now I had not bothered with it.