The Oxford Comma

I am a grammar snob. I am one of those people who takes photos of signs with misplaced apostrophes. And at some point during my formative years, I learned that serial commas were not just unnecessary but plain wrong. Then, some thirty years later, I read this in my Aspen Handbook for Legal Writers:

Although the final comma in a series (often called the Oxford comma or the serial comma) is optional in most writing, in legal writing it is required.

It is easy to see why if you consider this sentence from a hypothetical will, also from the Aspen Handbook:

I leave all my property in equal shares to Jim, Helen, Tim and Eva.

So, do Tim and Eva get a quarter share apiece, or do they split a one-third share? If there were a third comma, there would be no question. Now, after all those years of turning up my nose at serial commas, I experience a flash of cognitive dissonance every time I see a comma-separated series with or without the controversial extra comma.