a. How much tax evasion is carried on by cash payments? Certainly they aren't the only way people avoid paying taxes, and I would think that restricting oneself of bills, however high the denomination, would restrict one's scope.

b. I have a $100 bill in my wallet as I write. This is not because I have it in mind to help somebody avoid taxes on goods or services, this is because I missed whatever prompt on the Citibank ATM that allows one to specify "all $20s".