For me it just doesn't make sense with my crop frame 7D and the 17-55 which is effective 27-88. If I had a full frame camera, the 24-70 would have been my very first lens.
I agree also on what you say about the 70-200 taking great people pictures. I have taken a few and they are lovely. Two of the very first pictures I took with this lens were of my nephew and his wife in a cramped tea shop in Nongpoh on the way from Guwahati to Shillong. Even in that situation and with a hand held 70-200 they show what this lens is capable of.
I had posted them here earlier
My point is that if it was on a full body, I could get much more of what's happening in, say a bazaar, without having to go back farther to get everything in, if you get what I mean. I need something for that. I can do it with my 17-55 but then for a tight face crop its not long enough. When I walk around the city with just one camera I need something with the range of the 24-105. Actually ideally what one needs is the range of 24-105 on a full frame. That would be lovely.
Other than for this one use of just walking around, I am reconciled to carrying two cameras and with even my present three lenses (10-22, 17-55 and 70-200) I can cover almost all uses from landscapes and cityscapes to indoor sports, birds and wild life, assuming I get the tele-convertors Mk II when they are out next month for the latter uses.
Once I decide to get a full frame, as I may eventually do, I will discard my second crop frame (550D) and replace my EFS 10-22 for an EF 16-35.
I am also looking forward to one day shooting more with primes but I have to feel more confident about my photography before I do that.
Just now I am learning the basics :-)