Larry Summers wants to stop printing $100 bills. How about restarting the minting of 50-cent pieces? Former Treasury Secretary Lawrence H. Summers recently suggested stopping the production of $100 bills, ostensibly to deprive criminals of their favorite form of cash. Mr. Summers may or may not get his way. But here’s a potential trade-off: If the hundred-dollar bill is destined to vanish from cash registers, how about bringing back one of the jauntiest denominations of American spending money, one that many citizens, through no fault of