I have to say that the latest quote from AdviceToWriters is spot-on:
The use of commas cannot be learned by rule. Not only does conventional practice vary from period to period, but good writers of the same period differ among themselves. . . . The correct use of the comma—if there is such a thing as “correct” use—can only be acquired by common sense, observation and taste. SIR ERNEST GOWERS
Of course, try telling this to standardized test designers or curriculum experts.