The dyes used in color photography have absorption irregularities that give less than perfect color. They are not really noticeable unless copied where the problem compounds and degradation is then noticeable. That is why color negatives, designed for printing, are masked because the mask effectively cancels the impurities so they are not transferred to a print.
That's a fair point then. It would be interesting if someone had some visual sample to see the magnitude of the error. Did the photo companies or researchers ever release images that would show the difference?
Is the masking only important for convenient printing in RA4 type rapid processes with minimal opportunity to adjust the printing? What I mean is that in the past they didn't have a problem in commercial magazine printing when the source was a slide. But that is a complex process with various adjustment stages.