@ Seanahan – I never said the commas did disturb the flow; that was Mollusque. I agree with you that they don't.
You make a good point about Betty. Either way there could be ambiguity, especially if we replace "Catholic" with "Episcopalian" and "maid" with "writer". But it also matters whether or not we know the author of text does or does not use serial commas:
A. They went to Oregon with Betty, a writer and an Episcopalian priest. B. They went to Oregon with Betty, a writer, and an Episcopalian priest.
In A, they could have gone to Oregon with 1 (Betty, who is both a writer and a priest) or 3 persons, if we know the text does not use serial commas; but with only 1 person (the admirable Betty), if we know the text always uses serial commas consistently.
In B, they have definitely gone to Oregon with 2 people, if we know the text does not use serial commas; or with either 2 or 3 people, if we know the text always uses serial commas.
So both systems leave room for ambiguity. Life is like that. That is what house styles are for.
To Isoglossian, that Wikipedia page is terrible. They went to Oregon with Betty, a maid and a Catholic priest. They went to Oregon with Betty, a maid, and a Catholic priest. In the first case, Wikipedia says it is clear there are 3 people, but I'm not so sure, I could easily read this as an appositive. In the second case, there could be one person, Betty, or 3 people. I don't see how the comma effects the ambiguity at all. There are several easy ways to fix this by changing the words, but the comma does nothing.
To Rolig, I don't understand how "bacon, lettuce, and tomato" could possibly upset the flow of written text. People rarely if ever read the commas directly as pauses in speech, they let the flow of the words come naturally to them.
punctuation serves aesthetics, too. I find a serial noun phrase better-looking with an Oxford comma than without, and hyphenated compound nouns prettier than spaced ones.
as for the tempo difference between serial nouns with OC and without, it makes more sense to me to popularize the comma-free serial noun (often seen in cummings' poetry) to express that speedy reading.
Moll, it seems like you're arguing in favor of inconsistency, i.e. use the Oxford when you need it to clarify the meaning, but otherwise don't use it. And personally, that seems like a reasonable practice to me. As does its reverse: always use the Oxford except in cases where using it confuses the meaning. The only problem is that people usually want rules to live by, write by, and edit by, and the cases where the Oxford confuses things are probably much fewer than where not using it confuses things. So "always use the Oxford comma" seems a better rule than "never use the Oxford comma" and is easier to follow than "use it only when it helps" (because you don't have to figure out when it helps and when it doesn't). I like to avoid Emersonian hobgoblins myself, so inconsistency doesn't usually upset me if I see the reason for it, but for many people these little fellows seem to make good friends.
Rolig, that's my point. Because there is no consistency in the use of serial commas, most people read things the same way with or without them. This shows that they rarely have any purpose, so might as well use them only when they do have purpose, as in the example you give, which shows that Martin groups with Mary Jo, not Bart.
A writer who used serial commas only occasionally can use them not only to avoid ambiguity but to indicate emphasis or cadence. Using them all the time to avoid rare instances of ambiguity puts consistency ahead of expressiveness.
To me, the mandatory Oxford comma (as distinct from the optional serial comma), is similar to the American convention of always putting the closing quotation mark outside a comma or a period. Instead, I follow the British convention of put the quotation mark where logic dictates.
While I probably like commas more than most, I don't hold with the practice of putting them in just because you want to take a breath, or conversely, the idea the you have to take a breath whenever you see a comma. Not all commas are breath-taking. In the case Molly mentions, there is no need to "break the flow" in the sentence "Tom, Dick, and Harry all ordered bacon, lettuce, and tomato sandwiches." I would read this the same way with or without the serial commas. The commas before the ands are merely there because the writer (or editor) finds such commas useful and wants to be consistent. In this particular sentence the serial commas could easily be removed without the ceiling caving in. But I'd keep them because I like the principle of the serial comma. In the sentence, "Tom, Dick and Harry, Mary Jo and Martin, and Bart all went to Des Moines for Frank and Herb's wedding," the Oxford comma helps keep the relationships, well, straight (or not).
I generally don't use the serial comma, except when ambiguity might arise. Looking over my list of Triads, I see some where I should have included the comma for cadence, but many others I consider to be so closely associated that the comma is unnecessary.
I found one example from Wikipedia for avoiding ambiguity. Consider the following sentences: To my parents, Ayn Rand and God. To my parents, Ayn Rand, and God. the second comma before "and" in the second sentence should be the oxford/harvard comma~