Why get fast lenses?
Remember that AF is quicker and more accurate on many Canon bodies when the lens is f/2.8 or faster. So lots of fast action shooting benefits from a fast lens, even when the exposure is stopped down.
Also the subject isolation you can get with a fast lens is a special look - maybe a bit overdone for portraits, but I still like it enough to shoot a 135/2 wide open all the time.
Finally, some times you just need all the light you can get. I shoot nightscapes/starscapes at ISO3200 with a 24/1.4II near wide open. I guess if the 5D3 really has a clean ISO 12800 then I could get away with an f/2.8 - but I'll wait for reports before I go trading in all my fast primes...
Last edited by vachss; 03-23-2012 at 12:11 AM.