I know people that drive 100 mph on a regular basis also, but that doesn't mean it's a good idea,, and just because they haven't wrecked yet, doesn't mean they won't
I've read that advice in many places, it was the paper work for the lens that said it, not for a camera body..they also sell a device that supports the setup by the lens when wearing it around your neck,
not looking to stir anything up, but I don't think it's a good idea to discredit good sound advice just because you do things differently..
I often hear about people dying from driving insanely. I've never heard of someone's camera mount failing because they carried a mid-sized lens on it.
If you can show me where Canon has issued that warning for a lens in the size range of the 70-200 f/2.8 on a 1 series body, I'll give that some credence. Even better, if you can find me an example somewhere of someone that suffered from the practice, I'll be even more swayed. The lens is 1.45kg and my camera body is 1.2kg, that's not a huge difference (although the center of mass of the lens is farther from the mount, so it would exert more torque).
Canon has, I believe, the largest lens mount of any DSLR. The 70-200 f/2.8 L lens and their 1 series bodies are built to be extremely durable. I'm personally convinced that the mount is up to it. In fact, I'd do the same with my old 10D. It's only the polycarbonate body Rebel series that would give me pause.
I wouldn't mount on a tripod that way, but I've never heard of any problems carrying the lens around that way. The tripod mount issue isn't a concern for the lens mount; it's a concern for ease of adjustment and vibration reduction.
Don't believe everything you read. Some warnings are dramatically over cautious and some posters do insane things without suffering negative consequences. I didn't see my post as an attempt to discredit yours so much as an offering of an alternate viewpoint. While you seem bothered by it, I would have felt remiss if I let your original post stand as gospel without pointing out that I ignore the advice with no harm (yet). We're each free to make our point and each reader is free to decide what is the best course of action for them.
Now, I have seen one case where a 1 series body failed. It was with a 500mm f/4 lens (considerably larger than a 70-200 f/2.8). It was the result of a fall rather than someone holding the body. Before following the link, let me warn you that it is extremely graphic and disturbing. I hesitate to link to it because it's far to horrifying for a family site. I certainly won't put the image in my thread. This is not a photo that should be seen by children or the under-insured. I had nightmares for days after seeing it.
http://www.sportsshooter.com/port_popup.html?mem_id=296&i_id=587123
Don't say I didn't warn you.