Right. So considering that a person who would want the taxes raised on people of his income level would not raise revenue to his own acceptable levels if he voluntarily paid the extra money - that is, to the revenue level of his desired increased tax rate necessary for which he believes the government should pay for - why bother making this point?
It would not be enough but it would be something. Something is more than nothing.
Not to mention the principle. Put your money where your mouth is and so forth.