Re: It may be a US "standard", but...
You were taught it was wrong, but the teacher was wrong!
The serial comma (assuming you are referring to the Oxford comma) is necessary to delineate list items clearly.
Fred's favorite musical artists are The Rolling Stones, Simon and Garfunkel, and Hall and Oates.
Fred's favorite artists are clear:
The Rolling Stones
Simon and Garfunkel
Hall and Oates
In contrast...
Fred's favorite musical artists are The Rolling Stones, Simon and Garfunkel and Hall and Oates.
Fred's favorite artists are:
The Rolling Stones
Simon
Garfunkel and Hall and Oates
or is it
The Rolling Stones
Simon and Garfunkel and Hall
Oates
or perhaps
The Rolling Stones
Simon and Garfunkel and Hall and Oates
Or, of course, the answer we know to be the actual one, the one answer that is unambiguous with the Oxford comma. If you were not familiar with any of the artists in the list, you would not know which of the four lists were the real one. You could grok out that all three of the wrong answers require a band to call itself something clumsy, like "Garfunkel and Hall and Oates," rather than the preferred list form, but you don't really know that such a clumsy usage is not actually how the artists in question refer to themselves without having prior knowledge.
Of course, not all examples are as ambiguous as this one, but the Oxford comma always works, while the dropped comma relies on the reader to grok out that the last item is not a compound item containing the word 'and'. The comma makes it very clear and precise regardless of context, and clarity should be the purpose of language.
Biting the hand that feeds IT
