I can see many applications, especially in sports and action. I shot a lot with the 300 F/2.8 IS II last year, and definitely was aching for some extra reach.
No doubt a lot of people are, I was talking about the 70mm wide end - it's probably there to go along with Canon's new 24-70 lenses and to phase out the 24-105, but the 70-100 range will come at a cost and it's doubtful how many people will need this part on a tele lens... if you want flexible get a 70-300L as this is much more portable.
I shoot aircraft (often R/C) all the time, and they go from "dot in the distance" where proper framing means around 4,000mm to landing 20 feet away, which means more like 40mm. They can do this in a matter of seconds. Even at full-scale airshows, I usually use my 70-200/2.8L IS II with 2x TC III and when the planes land and taxi by the crowd, I have to take the teleconverter off and even then sometimes 70mm isn't wide enough.
So, yeah, the wide end would be most helpful to me, even though I'd use it at the long end most of the time.