Slate columnist Ben Yagoda recently wrote about a standard punctuation rule in American English: the placement of commas and periods inside quotation marks when those quotation marks enclose a title or some kinds of quoted material.
For example, we would write:
I love the song "Bad Romance," don't you?
I love the song "Bad Romance", don't you?
There are a few reasons for this preference. A primary one is that, long ago, American typesetters agreed that text looked better this way (putting below-the-baseline punctuation marks inside quotation marks prevents unsightly gaps in blocks of text). Another reason is mere tradition. This is just the way we do it.
In his well-reasoned post, Yagoda argues that this rule is illogical; he would rather that our periods and commas not intrude on titles like this. And his argument makes sense.
He also argues that a lot of people are already ignoring the rule when writing online. In the continuing language debate between prescriptivism and descriptivism, Yagoda is on, in this case, the side of descriptivism: that is, the "right" way to write is the way that enough people are doing it. (The definition of the word enough is a whole 'nother conversation; I'm not sure if enough people use its and it's interchangeably for a descriptivist argument that we should do away with the distinction, but I hope not.)
But "everyone else is doing it" is not a persuasive argument, and I know your momma taught you that. In the end, we, sadly, can't always let logic dictate how we write. By extension of the logic argument, we would spell words phonetically—English spelling is notoriously illogical and dependent on tradition, and this causes all sorts of problems and misunderstandings.
Here's the best reason to use standard punctuation as it's outlined by the major style guides, such as our dear Chicago: Punctuation shouldn't call attention to itself—it shouldn't get in the way of what a writer is trying to say. If a reader notices your punctuation, you're probably doing it wrong. If you often have to explain to people that the way you're punctuating something is not, in fact, an error, your punctuation isn't working for you.
I'd guess that the typical general-interest online reader doesn't know (or doesn't care about) where commas go in relation to a closing quotation mark. Seeing a comma in either place won't bother her. But people who know the rule will be momentarily jolted out of their reading every time they come across a period in the "wrong" place. Like them, I'll be happily reading along and suddenly interrupted by the thought "Typo?" flitting across my consciousness.
The purpose of grammar rules isn't to allow the people who know them to feel superior. (Most people get by just fine without knowing them.) Their purpose is to help writers express themselves clearly by using mutually understood guidelines.
Having standards (even arbitrary ones, like some punctuation rules and the spelling of a great many English words) allows writers and editors to concentrate on getting meanings across, without having to waste time wondering and debating about where the comma goes. The answer is right there, in Chicago. We don't have to worry about it.