Hi there,
there's one argument (in my opinion) against the use of a 16-bit input for mastering:
Since additional processing of the material will happen and this likely through some 24-bit devices,
additional dithering has to be done in any case. The mix you've got surely is already dithered.
Dithering always changes the sound of the material, more in a psychoacoustically than in a obviously
audible way. Some are talking about a slight veil that is added to the original sound, some about
an audible difference in the hi-mid to upper frequency range.
Anyhow, the thing is that double dithering (and this will happen in your case) will change the sound
even more since the dithering noise is added two times. It's not a must happen thing - but it can
lead to unwanted results. Some will recognize it, some won't.
But I think, your customer (in a bad case) maybe will complain about a sound that differs from other
masters he got - because of the fact, that he maybe has delivered 24-bit files to the mastering houses
the times before and now the whole thing sounds different somehow. Maybe this helps for negotiations
about the 24-bit files and yes, I think there's a huge difference in the sound of 16 and 24-bit, 24-bit has much more deepness and resolution.
Best regards,
Tannoy.