Homeguard
Points taken, but from a moral and professional standpoint, if I do a revalidation flight with a PPL and consider their airmanship to be unsafe then I will not sign their logbook afterwards. We as instructors have a moral obligation to highlight deficiencies in flying standards in order to safeguard certain pilots, their passengers and those on the ground who may find themselves in the path of plummeting wreckage!
Incidentally, I don't conduct revalidation sorties as a 'test', I consider it to be an instructional flight, and will give instruction in any weak areas present. If the revalidation pilot learns something during the sortie it's a bonus. I usually ask them beforehand if there is anything specific they want to look at, PFLs, stalling, whatever, that they might not have done for a while in order to take advantage of an hour with an instructor to improve their confidence and proficiency in those areas. If after an hour of instruction I still don't consider them fit to act as P1 on a light aircraft then I ain't signing, sorry...