Bigshot, are you asking me? I no longer print larger than A2. Most of my prints are A3 from 10mp cameras and with a 36 megapixel camera, I've never gone larger than A2 - yet.
EDIT: I presume you are going to prove that A2 is too small to show the differences between the 24-70 or 18-55 and a sharp/cheap prime you are probably right. But it doesn't stop the prime from being sharper at equivalent apertures. I frequently get requests for 15 megapixel images from my clients. At a sensor crop to APS-C resolution, the same goes true: the prime is sharper. At full resolution, the same hold true at 36 megapixels. The 24-70 is an excellent lens, but it is still less sharp. I've never had a client reject anything because it was slightly less sharp than another lens, but the fact remains that it is less sharp. Again, I don't personally care. If a client does, then that matters to me. I doubt they will as most resolutions they print at are A4 or smaller. They'd never be able to detect the differences.
But, the differences still do exist. If a person is after the absolute best, they should invest in prime lenses. Fortunately, lenses like the 50/2 and 28/2,8 exist, which are absurdly cheap even next to the 18-55 lens (on the used market). Long story short, I have no problem you proving that at viewing distances and at certain print sizes, it won't matter. That isn't what I'm saying. I honestly don't care. I shoot with the 85/1,8K with no worries at all and it is a soft lens. It's just that 24-70 is not as sharp as the lenses I mention, lenses which together cost less than 300$.
Edited by shigzeo - 2/14/13 at 10:40pm