It depends what format they were shooting.
If you shoot cropped sensor Canon 1.6x DSLRs you would generally avoid going smaller than f/8 if you are concerned with image sharpness and want to avoid diffraction blur.
If you shoot a full frame DSLR or 35mm SLR you can shoot at f/16 with essentially no visible diffraction blur in even a very large print.
If you shoot mediium format of large format then you can - and often must - shoot at considerably smaller apertures and in these formats diffraction blur won't be an issue.
In addition there are certain situations in which choosing a "less sharp aperture" can contribute to a photograph that appears sharper or plenty sharp. I won't go into the details here, but let's just say that a) the degradation of IQ due to diffraction blur at small apertures is a real thing, b) the point at which the diffraction blur becomes an issue is different for different formats, c) this issue must often be balanced against other issues that contribute to the overall quality of the image, d) the final form of the photo (web, large print, etc.) makes a difference in how these factors are weighed.
Understanding diffraction blur and other technical issues is important to producing photographs that have the aesthetic impact you are aiming for. It is not just a matter of "blah, blah, blah..."
UPDATE: Oh, God, I just read through some of the follow-up posts in this thread. Yes, indeed, this topic brings out some of the most ill-informed and bizarre opinions of any forum topic - well, maybe excepting the boring Nikon/Canon battles.
There is no connection between photosite density and diffraction blur. If you took a 12MP full frame camera and put a 50mm lens on it and made a photograph at, say, f11 and then repeated the process exactly except for using a 21MP camera, prints made from both photographs at a given size would exhibit EXACTLY THE SAME AMOUNT OF DIFFRACTION BLUR. (You would be right to point out that the higher photosite density photograph would produce a more accurate rendition of the blur, but there would be the same amount. To state otherwise betrays a misunderstanding of how digital sampling works.)
There are several possible reasons to want to increase photosite density in a given format. (Dealing with diffraction is not one of them.) To the extent that ppi is an issue with very large prints you could push the upper prints size boundary a bit more - but be aware that this requires extraordinarily good technique and equipment and that we are talking about very big prints. Most often it is not ppi/resolution that limits your ability to make a very large print - instead it is more likely that focus accuracy, camera stability, etc will limit you first.
Another potential benefit from higher photosite density is to create smoother transitions between tonal/color values.
It is important to keep in mind that to a certain extent this can become a "how may angels dance on the head of the pin" argument unless you make very large prints, use excellent equipment, and always shoot with impeccable technique. Oh, and make interesting photographs... ;-)
Dan
Something just dawned on me.
As I'm looking through my various photography magazines, admiring the
gorgeous landscape photos, I noticed almost every one of them has
been shot at apertures between f/16 and f/22. They are completely
sharp, beautifully composed, oh... and all shot by professionals.
Yet on these forums, everybody whines and complains about diffraction
at small apertures. "Your photos won't be sharp at f/22!" "Use a
wider aperture or it will be blurry!" Blah blah blah. Apparently, the
pros in these magazines don't agree. After all, they're out actually
taking photos (and getting paid for it) instead of pixel peeping and
coming here to whine about it.
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Insert obligatory quote here...
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G Dan Mitchell - SF Bay Area, California, USA
Blog & Gallery:
http://www.gdanmitchell.com/
IM: gdanmitchell
Gear List: Cup, spoon, chewing gum, old shoe laces, spare change, eyeballs, bag of nuts.
