Your 24-70 f/2.8II is an absolutely awesome lens. Why would you consider focal lengths that fall in the 24-70 range? My 24-70 f/2.8II is so good I've sold my primes as they were sitting unused. I'm talking about the highly regarded EF 24 f/1.4II and a Sigma 50 F/1.4. I don't miss either of them.
Shooting kids? You next purchase really should be a longer lens. You'll be familiar with the advantages of a zoom. Look at the 70-200 f/2.8isII. Plenty of photographers on the planet would name this as their all-time favourite, most used lens. It's heavy & expensive, but wow does it deliver...
I doubt there is a Canon pro shooter on the anywhere on the planet who doesn't have one, or have it on their shopping list.
Couldn't agree more. I might not be a pro, but I've been a parent for more that ten years now and zooms rule. The kids just doesn't move from your side to another, they actually move towards you and away from you also. No offense, but there can be a tendency to prime snobbery here. I can understand that in a studio environment or any controled situation primes are very good, but for many other situations I think zooms are great. Especiall when they are as good as the 70-200 or the new 24-70 MkII (haven't tried that one though)
I can't speak for anyone else, but here's why I want primes in addition to the 24-70 II (which indeed is a fantastic lens):
— More light gathering capabilities. I have a custom setting with a minimum shutter speed of 1/250 to freeze movement and prevent blur. When I'm shooting indoors, which is often, f/2.8 and 1/250 often yields an ISO that is fairly high. Even with the 5DIII's excellent high ISO, I prefer to keep it as low as possible.
— Shallower depth-of-field. I think super shallow DOF can be overused, but I like having the option of really isolating my subject from the background. See below for an example of a picture I took with the 85/1.4 a while back.
— Creative limitation. In my case, having one fewer choice to make (i.e. focal length) can lead to more creative compositions. This is why I sometimes prefer primes to zooms, even if the zooms are more flexible.
I am definitely considering a 70-200 II, but not right at this moment. I just don't think I'd use it enough, given my shooting style and my tendency to work very close to my subject. I'm not even sure I'd use a 135 much, but I'm going to give that a shot next. Who knows, maybe I'll change my mind and get an 85 and 135 in addition to the 24-70 instead of a 35 & 85. We'll see.