Here's an argument given to me by a lawyer (for what its worth):

Example:

No comma before the conjunction:
I gave my money to Tom, Dick and Harry.

According to a lawyer, if this were a very simplified version of a will, a lawyer would view 'Dick and Harry' as a partnership, therefore dividing the sum of money between two groups, 1.) Tom; and 2.) Dick and Harry.

By adding the comma before the conjunction, you are able to avoid any confusion in this area.

Also, when a series becomes complicated (i.e. ...Tom, Washington D.C.; Dick, New York; and Harry, Atlanta...) you must insert the comma. Again, it seems to me that using a serial comma makes the most sense.