In regard to the commas, this is restrictive versus nonrestrictive, right?

Yes, commas or the absence of commas will signal to the reader whether a phrase or clause is restrictive or nonrestrictive.

In the McPhee sentence, the absence of commas yields a restrictive sense, "daughter Margaret" indicating that this daughter named Margaret is signled out as one of a number. With commas, "Margaret" would be nonrestrictive, merely an appositive, a piece of parenthetical additional information. .

"He confesses that he writes sentences with arbitrary information, secure in the knowledge that the scrupulous fact-checking will put him right."

This makes me CRAZY! When you are the writer, you are the one closest to the information in the FIRST place. He's writing about Penn's family, and the daughter. Doesn't he KNOW if she had any sisters? If not, why not? If he did thorough research, wouldn't he know?

This reminds me of the story a colleague used to tell of the person who wrote a story about some Russia-related thing for New York magazine, and turned in copy that read: "There are TK-thousand trees in Moscow."

If you think it's important, look it up.

Frankly, I'm expecting magazines to cut way back on their fact-checking, and to simply focus on anything that might be legally problematic. So writers had better start being more proactive and responsible about what they put in their stories.

It's too much work for you to look it up? Don't write it.

When I was in college, I wasn't taught that there was such a thing as a fact-checker. The reporter was the person responsible for making their story factually correct.

Regarding that use of commas:
There is a movement (somewhere; haven't heard much about it lately) to eliminate the comma in such a usage, and to treat "sister" or "husband" (etc.) as simple descriptors, much like "editor" or "style maven," which go in front with no comma ("as copyeditor John McIntyre says,")

I have some sympathy with that. For the sense of understanding that sentence, does it *matter* whether Margaret had a sister or not?

But that's probably mostly laziness.

How do you feel about AP's command to use commas in John McIntyre, of Baltimore, instead of the much-easier-to-read-in-a-caption John McIntire of Baltimore?

I've edited with both preferences without losing much sleep over either. There is no grammatical reason to set off the prepositional phrase with commas, but I can uderstand the impulse not to make the place name look like part of the person's name. If I am John McIntyre, of Baltimore, I feel a little less like Charles Carroll of Carrolton.

"Without the commas, McPhee’s reader could infer that Penn had two or more daughters, with Margaret singled out in this case. With commas, the reader could infer that Penn had one daughter, with her name supplied as additional information. (The passage appears in McPhee’s book without commas.) "

Wow, I must have missed that day in comma school. I would swear I've never heard that concept before. It makes sense, though, when I think about it.

The commas that bug me most are the ones I think of as "audible commas," that is, they represent a pause in a hypothetical speaker's speech without being integral to the sentence's structure.

Perhaps they result from teachers helpfully suggesting that one put a comma where you'd pause when speaking.

These often end up mimicking unmatched commas, e.g., "John Smith of Bank of America, said in a news conference he called today, that they're doing great."

The McPhee article has also generated some interesting exchanges in the comments on this Language Log post:

http://languagelog.ldc.upenn.edu/nll/?p=1111

Maybe the readers don’t care where the commas go, but the lawyers sure do. Check this:

http://www.theglobeandmail.com/servlet/story/RTGAM.20060806.wr-rogers07/BNStory/Business/home

Excerpts below are all from this source.

“It could be the most costly piece of punctuation in Canada.
A grammatical blunder may force Rogers Communications Inc. to pay an extra $2.13-million to use utility poles in the Maritimes after the placement of a comma in a contract permitted the deal's cancellation.
…..
Language buffs take note — Page 7 of the contract states: The agreement “shall continue in force for a period of five years from the date it is made, and thereafter for successive five year terms, unless and until terminated by one year prior notice in writing by either party.”
……
Rogers' intent in 2002 was to lock into a long-term deal of at least five years. But when regulators with the Canadian Radio-television and Telecommunications Commission (CRTC) parsed the wording, they reached another conclusion.
…..
The validity of the contract and the millions of dollars at stake all came down to one point — the second comma in the sentence.
…..
“This is a classic case of where the placement of a comma has great importance,” Aliant said.

I use this case as a cautionary tale with my students. Especially when they say "who cares about punctuation? Everybody knows what you mean!"

One wonders about the comma in this phrase:

four joynted, strong fishing Rod

Is it a strong fishing rod that is four jointed or is it a four jointed and strong fishing rod, the latter needing a comma when and is dropped. And while we're at it, doesn't four jointed need a hyphen because it's a compound modifier?

Regarding not being the best judge of my work and needing an editor, my Darling Wife always seems to find the errors in things that I have written, read, and reread. Another reason I cherish her.