That's true--what I said was intended as an approximation. I should have added some "most"s and "probably"s.
I think we can say that if one or two other philosophers besides Plato think that something Plato says has value, the chances of it actually having value are higher than a random sampling of what Plato says having value.
Why then would historians emphasize the importance of primary sources? If you rely on a progression of secondary sources over and over, you get serious misconceptions. Such as, for instance, your notion of the forms as being indicative of a descartian mind/body split. That simply isn't what Plato wrote.
So, as it turns out, not everyone can or should read primary sources. It's only for the select few. But those that do won't be dumber than those that only read secondary sources.
I think we can say that if one or two other philosophers besides Plato think that something Plato says has value, the chances of it actually having value are higher than a random sampling of what Plato says having value.