I came across the following perfect example of the importance of the serial comma, in a ProPublica article describing a problematic data leak:
The story prompted a leak investigation. The FBI sought to obtain my phone records and those of Jane Perlez, the Times bureau chief in Indonesia and my wife.
Under the serial comma convention, the phrase “Jane Perlez, the Times bureau chief in Indonesia and my wife” describes a single person under three descriptions — a proper name, a professional post, and a personal connection. With the extra comma, “Jane Perlez, the Times bureau chief in Indonesia, and my wife” would describe three distinct people.
Unfortunately, since it’s not clear whether ProPublica always follows the serial comma convention, the version without the extra comma is ambiguous. (I even looked for a ProPublica style guide that might clarify their house style on the matter.) For that reason, when I first read the sentence, I actually could not easily determine whether the one referent or three referent interpretation was intended. The solution for the careful writer: Always use the Harvard comma where appropriate, and in cases such as the above, rephrase the sentence to disambiguate:
The FBI sought to obtain my phone records and those of Jane Perlez, who is the Times bureau chief in Indonesia and my wife.