Point taken, but OTOH, the "rule" that if you drop it, it's garbage, removes any common sense, subjectivity, or inspection from the equation. If I have a garage floor like an airplane hangar, and I just mopped it before my detailing session, and I look at my clay, decide it's clean, rinse it off for good measure, what is my risk?
I think the same logic can be applied in reverse (sort of). If you drop your clay substitute on the ground, which is made of very soft rubber, and you just rinse it off and continue using it, are you absolutely sure there is no grit that has embedded itself in the rubber and didn't rinse off? (Disclaimer: I haven't used any of the clay substitues). Do you do a thorough inspection?
As I mentioned before, we've all clayed cars that have tons of tar, with pieces of aggregate, and that junk all gets stuck in the clay. Personally I rub the clay with my finger in my rinse bucket, and pick off any large particles, before I knead the clay in those circumstances. Am I removing 100% of the grit? No. Does that stop me from kneading the clay and continuing? No. What about you guys?
I don't see that as being hugely different from dropping the clay on the ground. You've got to use your best judgment (and I understand that newbs may be reading this without the experience to make that judgment).
Who's going to start the Garry Dean Clay Method (GDCM) where as soon as the clay gets dirty you throw it out instead of kneading it?