Whirly, you wrote: So while it may be good and desirable and sensible for pilots to do regular flying with an instructor, that has nothing whatsoever to do with whether it's legal for them to do a "check" with a non-instructor, or who can distinguish between rustiness and a pilot who shouldn't be flying. And I'm an instructor, and can therefore do checkrides, and I'm damned sure I wouldn't be able to tell the difference in all cases.
If you, as a FI cannot tell what is safe and what is not, I'm equally 'damned sure' that a totally unqualified non-instructor has even less chance of ascertaining the 'rustiness' of any pilot they might be 'checking'.
Using non-CRI/FIs for any form of 'check' is totally unacceptable. That's the way bad habits get passed on. PFA coaches are all now CRIs at least, so for them to do such flights is fine. But for the CFI's best non-CRI/FI drinking mates to do them? Absolutely not!!
Those who use 'trial flights' as simple revenue gathering joy rides must remember that the CAA keeps a close eye on such things; however, if any PPL non-FI conducts these flights then both the club and the pilot deserve all that the Enforcement Branch can throw at them. Perhaps an 'undercover' visit to the place in question will be forthcoming very soon?