FWIW, I used to shoot primes exclusively. I had a 35, 50, 85, 135. I was mostly doing fine art and street stuff, but 2.5 years ago I had a kid. Now I've sold all of the primes except the 35 and do 90% of my shooting with the 24-70 II and 70-200 IS. Since my 2.5 year old daughter rarely stops moving, f/2.8 is plenty wide enough in most cases, and the convenience and flexibility of the zooms outweighs the extra light and shallower DOF of the primes. And as others have pointed out the IQ, AF speed, flare control, bokeh, etc. of these new zooms is so good that I don't find myself missing the primes for that reason.
I still use the 35 (Sigma Art) for indoor work and when I don't want to lug one of the zooms around—though truthfully the size/weight difference between the Sigma 35A and 24-70 isn't that significant. I am considering trading the Sigma 35A for the 50A when it comes out, because I think the 50A might be more versatile as my single prime.
At some point when my daughter gets older and stops moving so continuously I might reacquire some primes for more deliberate work. But right now the zooms suit me best.