A recent paper from Harvard's Kennedy School of Government argues that all large denomination notes like the $100 bill should be eliminated, as they enable criminal activity across the world. This is because, according to the study abstract, they are anonymous, lack any transaction records, and can be moved and transported with relative ease. Eliminating the $100 bill, as well as the €500 note and the £50 note, would go a long way in frustrating money launderers, corrupt officials and other less-than-legal entities, said the study.

"Without being able to use high denomination notes, those engaged in illicit activities – the 'bad guys' of our title – would face higher costs and greater risks of detection. Eliminating high denomination notes would disrupt their 'business models,'" said the study abstract. 

 In other words, it's easy to transport a million dollars in $100 bills. It's much harder to move that amount in, say, $20 bills.