To answer the topic question of why primes is that Canon still makes such fine primes lenses that their IQ is just breathtaking when I have done all things right. I first bought zooms and I was happy. Then I needed a macro and got the 100mm L with IS. I dared to use it for normal shooting and not just macro. Something seemed different in the images -- better? I had the 100-400 but wasn't really happy about sharpness at the 400 end. It turned out I needed a better tripod and head as well as better technique but I first bought the 300 F4 L and a 400 5.6 L. While they were good (better than the 100-400) I lusted for more so I bought the 500 F4 L just a month before the prices soared earlier this year on all the super teles. I want the 400 2.8 L II but the prices are just now in the stratosphere. I'm very happy with the 500 L until it broke. Canon fixed it but now I'm always worried about it going bad again (loss of focus).
Since I always have had the Canon 50 1.4 and the Canon 85 1.8 from the beginning of my digital conversion and since then I've bought so many lenses at this point, I started a large project in defining a test senario in testing all my Canon cameras and all my lenses regardles of make. I'm mostly a landscape and seascape photographer making large prints. I choose one location to do all my shooting and one rock in the scene to set my focus point. All shots used live view (10x for manual focus, no IS) with a cable release and a Gitzo tripod and RRS head. I shot from wide open with each lens and every f-stop available and at all major zoom settings for zoom lenses. All shots were at 100 ISO. I shot with a 5DMK II, 7D, and now a 60D.
What I found is that the absolute winners in the tests were my tilt shift lenses regardless of camera body. The tilt shift lenses had great shapness edge to edge including all the corners. The tilt shift lenses were the 17 TSE L, and the 24 TSE L II. The next best lenses were the primes like the 24 1.4 L, 35 1.4 L, 50 1.2 L, 50 1.4 (non L can win), 85 1.2 L, 100 2.8 Macro L, and my new favorite 135 F2 L. I'm considering the 14 mm 2.8 L but I'm not sure yet.
Next came the zooms and I the ones I really found great was the 70-200 F2.8 L II and the new 70-300 L. The new 70-300 L is just so much fun to use and so much lighter than the 70-200 that zooms are fun again.
As others have said, the primes are always better indoors because they are faster. Also better indoors because they are black and smaller and draw less attention than the big white guys not to mention that they are much lighter and easier to work in a crowd.
Since I have the flexibility of putting any of these lenses on the 60 D (which I like the noise pattern better than my 7d) or the 7D, the 135 F2 is now 216 effective mm and 300 if I put on a 1.4 extender and the lens with extender is still 2.8 at far less weight than the 300 F2.8. Of course the 300 F2.8 has IS and that might be my next wish list lens but I'll wait until the price stablizes (if it does). I'm concerned over the value of our dollar. Will that lens also be put into the over 10K limit that I've set for myself like the other super teles? I hope this rapid price increases on the super teles stops next year.