As with other paired bracketing devices (such as parentheses, dashes and quotation marks), there is actual mental cruelty involved , incidentally, in opening up a pair of commas and then neglecting to deliver the closing one. The reader hears the first shoe drop and then strains in agony to hear the second. In dramatic terms, it's like putting a gun on the mantelpiece in Act I and then having the heroine drown herself quietly offstage in the bath during the interval. It's just not cricket.