The 2-yearly JAA PPL revalidation flight (or whatever it is called) seems to do nothing. The instructor has no power to fail the pilot and more or less must sign his logbook - so long as, presumably, the instructor has survived the flight.
This has come up on some pilot forums from time to time and I gather that originally this flight was framed as an FAA-style BFR (which you can most definitely fail) but there was such an uproar that this was dropped. IMHO, loads of pilots would fail a "BFR" and even just from flying around it's obvious that many can only just about make a radio call despite having been flying many more years than I have.
The BFR works well for the FAA which directly or otherwise runs at least 90% of the world's GA.
Mind you, there are cowboys everywhere and you will never weed them all out. This kind of thing is a personal tragedy but it's also bad news for all of UK GA because it ensures that the separate debate of "what expectation of safety do passengers have" is kept in a high profile and this in turn maintains a huge raft of Euro regulations concerning parts certification, maintenance procedures, etc. - much of this stuff is totally irrelevant to safety in GA but it keeps the costs up.
It's obvious that somebody climbing into a C150 should not expect the safety of a 2-crew 747, but where do you draw the line? You certainly don't expect to end up with a cowboy.