Unless you were born in the 1930 thru 1940, no one carries different denomination rolled up in their pockets.
Real practical problem for people like myself who carry very little cash and live on ATM. You don’t need cash much these days and wait Smart Phones (iPhone, Droid) is going to have Cash, and Credit cards soon and be able to make person to person payments. Some banks like BoA and Chase already has Person to Person payments but not Smart phone based though.
Payment revolution is on the way folks, embrace and enjoy spending money.