The input capacitance acts as a load to high frequency input signals so, if you are trying to convert a high frequency input, you want minimum input capacitance to minimize the requirements on the source that provides the signal to the ADC input.
I think the problem is semantics. I took the OP's statement literally and though he was referring to the intrinsic input capacitance, but you inferred that he is talking about adding a small input capacitor. You probably are right. :-?