I have to say that I don't know any pros using the 100 - 400mm IS L. Most commercial portrait & wedding work stops at 200mm, I'm unusual in having a 300mm f/2.8, most times a longer lens is needed it's so rare that it's off to the hire guys. At the other end wildlife & sports, they all seem to be using the big white primes. I've yet to see a press line up of big lenses where one guy has brought his 100 - 400mm it just doesn't happen.
That's exactly what I mean. These compact (relatively) lenses with wide zoom ranges appeal to hobbyists who can't justify the cost of faster lenses, but they are not suitable for pros. I have never seen a pro using a 70-300 or 100-400 lens but loads of hobbyists seem to have them. As for the 400 5.6, Canon also make a 400 f/2.8 and If I was making my living from images in that range I would buy the 2.8 as it would soon pay for itself.