Many of the photography enthusiast websites out there are run by old-world photogs. These are guys and gals who have been in the business for over a decade, and in some cases, over multiple decades. As such, they have a lot of peculiarities unique to them. A rabid love of film. Near constant qualifications in their articles about how images on a screen pale in comparison to those in a high-quality print. A love of Leica, regardless of how gobsmacking stupid their prices get. And, notably, frequent talks about how the sharpness of a lens doesn't matter.
They'll say that all lenses are the same once stopped down. They'll say it's the composition that counts and that any differences can only be seen on HUGE prints. They will, of course, say this right after extolling the virtues of how sharp Leica's lenses are.
Well I want to call bullshit on that. I've never worked in film. I'm old enough to have done so, but I only became interested in photography after the release of the EOS 20D and have been digital ever since.
Yes, if you take a photo with two lenses of the same length, print the images, you will likely not notice much of a difference unless the prints are very large. But that only works if the photo you have taken is perfect. As is. The one you want. Oh yes, I'm sure all of the photogs writing these articles are of such immense skill that they never need to crop in post, but for us mortals, the sharper the lens, and the larger the sensor, the more you can crop out and maintain maximum detail. The instant that your cropping exceeds what you wanted for your purpose, you've exceeded your equipment. And I crop almost every photo that I take.
For example, you need a 300dpi print that's 10"x10". That's 3000 pixels square. Aka nine megapixels. On many modern cameras, that doesn't leave you a lot of crop room. The new Olympus E-5 is only 12Mp. The Nikon D3s is the same. If you're using one of the lesser lenses, you have very little wiggle room from which you can extract professional-looking shots. You want the sharpest lens possible because that, like digital itself did, frees you to be more spontaneous in your shooting. You don't have to worry about composition or focus, you can aim your camera and start clicking off shots.
A few provisos, here. One, this doesn't apply to landscape. For obvious reasons. Two, Leica's lenses really are amazingly sharp. Their prices are just so fucking comical that only an idiot buys them. This applies to people who use a lens for a wide variety of applications. I rely almost entirely on a 90mm lens that's sharp as a tack at F2.8 and a 40mm lens that's sharp at F1.7, and really sharp at F2.8. I always have at least one of them. I use the F1.7 lens the most because I can jump from full-sun into a poorly lit room with the same lens. That is critical, and having that lens be as sharp as possible means that the raw material from which I craft photos in post is as good as possible. I want my lenses to be sharp enough to cut meat.