You should read the latest edition of 'GA Feedback' which arrived on my doormat a few days ago. There is a report on just this subject. The Head of FCL writes:
"The AIC emphasised that the flight did not represent any form of proficiency check although the instructor was able to use his discretion and refuse to provide the required signature in the log book if he felt the pilot had not demonstrated a safe level of competence."
So we're only signing to say we were there, unless we weren't happy with the standard, in which case, technically, we weren't there!! Without the signature the licence cannot be renewed for another 24 months unless he passes a separate profieciency check or finds someone who is prepared to sign him off.
Sorry, but in my book instructors are being set up as pseudo-examiners upon whom someone's licence renewal can hinge, and that's not right.