Hmm, that's a good comparison. Why don't I dump my DSLR and drop $27k on a medium format digital, which will probably max out at ISO 400. Not exactly the right tool for the job, donchaknow? Top-notch for studio or landscape stuff - lousy for demanding Disney park photos.
Insert about 5,000 "roll eyes" smileys here. Please stop trying to pull brand into a discussion where brand is not an issue. Again, try reading some lens reviews. Zooms almost always lag behind primes in terms of the issues I stated, to one degree or another. It has nothing to do with who makes the zooms or who makes the primes. Look at any review. Heck, I just read Pop Photo's review of the Nikon 24-70mm F2.8 - a $1,700 lens that they thought was "optically flawless", and they do say that it has visible barrel distortion at the wide end, pincushioning on the tele end, and vignetting when wide open. These are issues that would be minimized with a prime.
Anecdotal stories of using old lenses on film shooting daytime parades are completely unrelated. It's much easier to see lens flaws with digital and daytime parades are hardly challenging for any camera.
I haven't noticed the issues significantly but I very rarely make prints (though I am probably going to start soon) - I also value "real world" testing however the fact is that lab tests will show issues that may be harder to quantify in "normal" prints, but help explain why certain lenses produce photos that "pop" more. A mediocre lab test is also a pretty good indication that the lens is not going to be a great performer.
Bah, again with the "pros". Tell a "pro" to take his sports or paparazzi gear and try to get a good shot of the floating Madame Leota head. A "pro" will go in when the ride is stopped, turn the lights way up, and use a tripod. (Just look at any "official" on-ride photos, which are much better-lit than the actual ride.) As amateurs, we don't have that luxury. And 1600 ISO and F2.8 just aren't enough (especially from a moving ride vehicle) and most cameras don't produce clean-enough 1600 ISO to allow you to push it much further.
Again: two extra stops. That's the difference between F1.4 and F2.8. That's also the difference between ISO 400 and ISO 1600. Or the difference between 1/50th and 1/200th of a second. You can do what you want - but when the lights get dim at Disney - I'll take those two stops, please. During the day, sure, F2.8 will be fine, though I'll still probably be using the primes a lot.