This claim is frequently used in association with Playboy but not often with "other" "pornography" magazines. Why?
Most people who haven't actually paged through a copy of the magazine (women, religious officiaries, teachers, boys before they find their friend's father's collection) think the line is a joke and is funny. Those who have seen (and usually read) a copy realize that it's a perfectly legitimate explanation.
Playboy is not a pornography magazine.
What you say!!
You heard me right. Playboy is a magazine for playboys. That is, typically wealthy, conservative, white, anglo-saxon protestants who try to "live it up" by playing golf every day in their oceanside country club and dress up at night with an ascot and drink fancy wine from a particular year1. This magazine caters to them.
Playboy has articles on things which are socially "neat" (sometimes bordering on "trendy" -- though bordering on the cutting edge or trailing edge varies). It's full of advertisements for top-shelf alcohol, big name cigars and sports cars. Then, somewhere in the middle and usually one third of the way to each cover, are some pictures of naked women.
Literally, these are pictures of women who happen to not be wearing any clothes. The poses aren't much different from what you'd find the models doing for a cologne advertisement, they just happen to have their breasts and vulva showing (sometimes). These pictures, after being shot and developed, are airbrushed to remove any imperfections that the caked-on makeup didn't cover. In the end, there's more paint between the viewer and the model's body than there are clothes on normal advertisement models.
Young boys who first discover nudity through Playboy are fascinated at things they haven't seen before. To everyone else, it's just naked women. To playboys, I imagine, it represents everything else in their life -- perfect beauty after which they can lust.
In the end, the bulk of the magazine (as is the case with most magazines) is cultural grooming in an attempt to sell products or services. Secondary to that is the latest chapter from a Chuck Palahniuk book, and tertiary is a collection of doctored photographs of some attractive women who happen to not be wearing any clothes.
Some people do "read it for the articles." But most people don't bother with it at all.
1 Yeah, I'm going overboard in my stereotype, but it's for building the image. | 
