YAY - super long post that no one will read!
TLDR: Wash your hands and stay away from crowds. And eat your vegetables. Bye!
This isn't exactly an exhaustive dissertation on the topic, but I ran across this video which brought up a couple of salient points:
First of all is the reiteration of what I've said before about how DPI is for printing purposes only and has no relevance to digital resolution. Also, the fact that adequate resolution is directly related to the size of the media and the distance from the viewer (in a magazine's case, around A4 size held slightly closer than arm's length.)
Also, as I said before, when we read magazines on a computer monitor, it is typically further away from our eyes than a handheld magazine page would be, so we usually view the digital image at a size larger than the dimensions of the physical magazine to compensate.
The interesting points in this video relate to how - the larger the image we're viewing is (and thus, the further from it we typically have to view it to see it well), the lower the resolution can be with absolutely no perceived lack of quality. As pointed out in the video, a billboard is printed at 15 DPI. It COULD be printed at 300 DPI (or as some of you would prefer, 600 or higher DPI) but the only way anyone would be able to notice the improved resolution would be if they were so close they would only be able to see a tiny portion of the billboard (which is something that no one should ever need to do.)
It then points out that a glossy magazine is printed at 150 DPI. I'm not sure if this is true of all magazines, since I've seen other sources say magazines are printed at 300DPI. It may depend on the magazine. Regardless, it does suggest that scanning at 600DPI isn't really necessary to capture all of the actual printed information in a magazine image (though for editing purposes, larger is easier, so I still scan at 600DPI.)
So again, this argument boils down to two ways of thinking:
1. Magazine scans are something meant to be read. 300 PPI is more than large enough.
2. Magazine scans should be 600 PPI or higher because they may be used for purposes other than reading.
Proponents of #2 have yet to explain exactly what other purposes they have in mind for a scan that's over twice the resolution the magazine was printed at would be...
So far as reading resolution goes, here is a more thorough explanation of why I believe that 600ppi is overkill:
Most of us read magazines using CBR readers (though some of you prefer PDF). CBR was designed for comic books, which often have images and captions spread across facing pages. Rather than having each page saved separately, a two-page spread is saved as a single jpg. Thus, a digital comic book file will be composed of a mix of single page jpgs and double page jpgs. When reading the comic, scrolling down a single page will go from top to bottom. Scrolling down a double page will go from left to right, then down, then left to right again (like reading lines of text in a book) so that the reader can read the panels in a two page spread in the proper order.
Text in magazines, however, almost never stretches across two pages. Thus, when we edit magazines, we do it with single-page viewing in mind. Any two-page spreads need to be edited so that they match up if the reader is using two-page view mode, but each page is saved as a separate jpg so that the entire magazine can be read one page at a time. Some people may VIEW the entire mag in two-page viewing mode, but by design they must READ the magazine one page at a time.
OK, so a modern HD display is 1,920 px wide x 1,080 px high. When we view a magazine on our computer, we are often zoomed in, so we would want the scan to have a larger pixel height than just 1080. However, ask yourself how many times you've read a magazine where the width of a single page didn't fit on your display. Meaning - when viewing a single page, you had to scroll to the right to read a single line of text before continuing down the page. Literally NEVER, right? Of course not. Which means that you would never have any page displayed at a size larger than 1920 px wide. And I don't know about you, but if I have an image set to fill the entire width of my screen, the field of view is much too narrow, so I ALWAYS have the page displayed smaller than that. Well, a 600PPI magazine page is about 6400-6500 px wide. So if you were trying to read a mag at that resolution, you would only be able to fit less than one-third of the width of the page on your screen at once. You'd have to scroll the entire length of the displayed image to the right 3 times to read a single line of text.
Another way to look at it would be to consider the pixel area. An HD monitor is 2,073,600 pixels squared. A 600 PPI magazine page is 31,200,000 pixels squared. You would only be able to see 1/15th of the page at any given time. Imagine that. Cut up a magazine page into 15 equal parts, arrange them in order and try to read them. Impossible. No one would EVER (could ever) read a mag like that. It's just WAY too big for the purpose it was created.
And for anyone who's chomping at the bit to say "but we're just future-proofing everything for our 16k displays 20 years from now" I'd direct them to the top half of this post and ask them what good comes from saving the files at a higher resolution than the source material was printed at. Photos are one thing, since real life comes in...really really high resolution. So bigger is always better (though as pointed out above, it's also unnecessary unless you're trying to view something super close). But magazines are printed at relatively low resolutions, so scanning them at 1 million DPI doesn't serve any purpose if there are only 150-300 dots per inch actually printed on the page.