Outside of a class 10 clean room, no optic in the real world will "pass" this test.
Not true. The mirror that came with my RCOS 12.5" RC certainly passed the flashlight test, as have at least two SCTs I owned in the past: one from Meade and one from Celestron. In addition, I had to send a C9.25 Edge back for repair and I asked the service tech to clean the mirror (which came, brand new, with a coating of grunge on it). When the C9.25 came back from being repaired, the mirror was clean as a whistle and passed the test with flying colors. It should have come that way in the first place.
Consider what it does take in order to impair the image when looking at the central obstruction's contribution.
Any optics will look horrible under those conditions. Leading the owner to clean and clean and do more harm than good.
I may be many things, but a clean freak just ain't one of 'em. Just ask my CEO.
I know that discreet, or focal, defects do not matter. We've all seen or read about a large chip in the primary mirror that, when painted black, had no apparent effect on the scope's usability (although, depending on the size of the defect, it could reduce the effective aperture). And a few sleeks, or a few specks of 'whatever' is not my concern.
The central obstruction is not a flaw involving the primary mirror, so it's effects on contrast may not the same as light scatter off of the primary mirror due to a coating of dirt on the surface of the primary mirror. Or if it is similar, the effects of image degradation are additive - and that can't be good.
What I have discovered from doing many flashlight tests over the past 30 years or so is that most of the time they uncover meaningless, extremely minor defects. (In them thar cases, Unk, ya'll are right on. Clean freaks will do more damage trying' to rid the mirror of them inconsequential flaws than what the dang defects cause.)
But I have certainly had CATs whose mirror surface - or at least half of the mirror's surface - was coated with a very pale gray-to-yellow cast caused by who knows what. That coating of crud only showed up during the flashlight test, so the owner of such scopes would not know their mirror was dirty unless they employed the flashlight. It's those instances that concern me.
We were recently treated to an Excellent paper "MIRROR VS. DIELECTRIC VS. PRISM DIAGONAL COMPARISON," by our friend Bill Paolini. I think is clear to say that Bill proved rather decisively that light scatter off of dielectric coatings can degrade contrast enough to hinder planetary detail... and them were clean diagonals he tested, Unk. So if light scatter from dielectric coatings can degrade planetary detail enough to make many of us buy new prism diagonals, are you guys who are dissin' flashlight testing of the opinion that mirror grunge has no such deleterious effect?
I would encourage the use of flashlight testing - with this caveat: I'd ignore the minor stuff, but when the flashlight beam reveals a large area of grunge on the mirror (grunge that shows only during the flashlight test), I'd send the scope back (under warrantee) for a cleaning.