I wasn't commenting on whether instructors would know this stuff, rather aspiring PPLs will not give a stuff about it and just want to learn how to fly, 44 pages is a decent read for anyone that's not very academic in nature. If one of them came to grief post training, or during training the court system will not really care for the signed documents as the trainer can not put the onus on a trainee to learn something that they have contracted the trainer to teach them, ie you have not signed up for a self education course when learning to fly. In almost all situations, it's the trainer and checkers responsibility to ensure the standard is met, both in training and on check day. Where it gets tricky is when a student comes to grief and is seriously injured or dies and the family sues the flight school for compensation, at this point it will be scrutinized as to whether the student was competent to be conducting that solo exercise, signed documents will only go so far, but will provide some insight into the schools process. Now in the states some public cases have gone through where student pilots have tried to sue the instructor for damages post an accident. Usually the court will rule on the outcome of the accident, that is, if they successfully employed some technique or procedure and survived the trainer/checker are absolved.
The purpose of signed documents in flight training is more that the student has read and accepts the outcome of the particular lesson. Then if it's disputed later if the student over runs hours the school can go back and say, 'on several occasions you were told you needed to improve xxxx, and you put in little effort, so your progression was hampered by your own attitude'. If you went back and all the documents showed progress with no notes as to why sequences were repeated, you could be in for some refunds...
Taking the Little Johnny example, if neither he nor the instructor are aware that this is what is actually meant by "competent in land aeroplane" ie when he can consistently do all of the things in this unit then you are right the student records become meaningless and belive me I get some meaningless tick.n.flick student records forwarded to me often and wonder what on earth the HOOs are doing.
Performance criteria for a sequence should be briefed prior to the flight, hence the old Aim/Objectives of a mass/pre-flight brief. It should be clearly spelled out at that point and discussed as to what you are trying to achieve and to what standard is required to pass. This should be covered for all sequences of training. If instructors are not adequately briefing students pre-flight then I can understand now why average solo times are ballooning to 10-20 hours.
I agree that a student should have access to the MOS, and standards by which they will be assessed, but this does not mean the onus is passed from the instructor to the student somehow being able to interpret these documents. And most importantly the MOS does not give an instructor the techniques to be applied, just what are the units to assess competency.