Nzapanda, your opinion, and, correct me if I'm wrong, is based on a grown adult thinking rationally about riding and finding that rationality and a responsible approach can, indeed, lead one to start on a larger motorcycle.
For the most part the rest of us are thinking about far less mature riders lacking the self control, life experience, and brain development of an adult...never mind the testosterone level differences and susceptibility to peer pressure and the gee whiz nature of riding.
Like Richard I started in the 70's as a teen on a small (in my case 50cc) motorcycle. I am very glad I did not start out on a Triumph Bonneville, (slower than modern 650's), as I would, no question, have done severe harm to myself. As it was I crashed that road motorcycle several times and even managed to get hit broad side by a car in a suburb of Brussels.
Yes, today, I might do better as there is actual training and information out there to help new riders that simply did not exist in the 70's so, perhaps, starting on a 300 would be prudent today.
If my now almost 30 year old daughter had wanted to learn that is the path I would have followed; MSF prior to buying a bike and buying one of not more than 400cc to mentor her on.
Why do I say 400cc?
Because, though I've owned many bikes, including a 1000cc SS, no motorcycle I've ridden is as "easy" to ride or as fun to ride as my DRZ400SM. I'm certainly not slower on the street than I was on my literbike, and, in fact, I dumped the literbike once I had the 400cc SM, (though I do also have an 800cc sports tourer for longer jaunts).
I'd still recommend that course for more mature riders to be honest.
If you had a teenage son who wanted to ride would your really recommend a 650 to them?
PS: yeah, buying rentals in the downturn was the best investment decision I've ever made.