I'm checking out the MTF charts, and there does not appear to be a significant sharpness edge from using only prime lenses. Is reality misrepresented on the MTF charts? Are they not that much better than zooms?
Sometimes, 70-200II is also sharp but the img is not smooth and natural enough.
Primes, especially L class are sharp for the reason that it is more accurate in optical design.
Sharp is only one part, for primes, the unique feeling makes the difference.
Eg. 35L is rich in color and sharp enough even at F1.4
50L is not that sharp but is full of environmental feeling at F1.2
85L is extremely sharp at F1.2 and the bokeh is that nice...
This summarize may not be very accurate and complete but hope it could help understand
Environmental feeling - what?
I think zoom lenses are catching up and soon there will be no significant difference between a zoom and a prime (yes, they will be faster, but who does really need f/1.2?). My 70-200mm f/4 is extremely sharp, has great color and awesome contrast and costs less than some primes. So, I don't see the advantage of prime lenses at all (unless you are a professional photographer and need super sharp corners)...
Uhm..... f/4 is EXTREMELY slow of a lens. It's a death sentence for anyone who wants to work in low light. f/1.2 lets in almost 10x more light than f/4, think about that.
There are countless examples of why primes are advantageous over zooms for image quality. Generally sharper, better contrast because of less glass in the way, lighter (usually), smaller, allows for slower shutter speeds when required. Also, the low light abilities. Try taking f/4 to: a concert, wedding, event, travel, anything remotely dark. Even in some dimmer day time scenarios where you want more speed. High ISO isn't going to save you. Not to mention more control over depth of field. This is a big one. Having the ability to choose more options when depth is concerned, is also a big plus. Who wants less?
Besides, sharpness is only one factor in making a good lens, and it's not even near the top of the list... but primes still win there, mostly.
I shoot at f/1.2, f/1.4 and f/2 all the time. Daily. Even with my film cameras I do. It's a necessity for a lot of people, and even if it isn't REQUIRED in some situations, having the ability to control the depth of field is an INCREDIBLE asset. Sure, you can get good depth of field blur from 70-200mm f/4 lenses, but you have to be pretty far away, even to frame closely at 200mm. My 50L, or the 85L, I can be quite close, and have insane depth of field control.
To someone who spends his life taking photos of people, that's absolutely a key factor. But regardless of that, I find that most people drool over these lenses because of that, because they CAN achieve such results, even if it's a snap-shot, or a purely hobby sort of thing.
Once you handle all the classic L primes from Canon (or I mean, a bunch) you start to see exactly what I mean. They are special lenses, and it's FAR more than just sharpness that makes them special. They produces extremely amazing images from an over all feel stand point. Once you know, you'll never forget.