I never said any of those advantages are not apparent, I specifically said that speed and ease of use are much better with a built in TC.
I think we are not communicating clearly on the getting the shot part. I 100% agree that getting the shot is fundamental. The typcialy use case for the 200-400 without a built in TC would be:
1) Observe that subject size and distance (shy rhinos on safari) for todays shooting requires TC, put on TC.
2) Spend some time shooting distant rhinos on safari
3) Suddenly a large male lion approaches landrover very closely, zoom out and capture identical photos to that guy with the lens with the built in TC but accept shooting at f/5.6 or less vs f/4.0 or less. Guy with fancy new Canon lens just has to switch theTC setting on lens which is nice, but when comparing photos with a G&T back at camp there is relatively no difference.
No, Guy with TC on has to quickly try and change TC so he can get a 200mm focal length, not 280mm, or he shoots with TC on and zooms out as much as possible only getting the face at f/5.6. Guy with switch flicks it and gets 200mm f/4 and a shot of the entire lion, or zooms in a little and gets face shot.
4) Next day on safari, flamingos spotted but are a long way form shore. Guy with plain 200-400 puts on 2xTC, AF is slow and the images wont be as crisp at a naked lens but he gets some nice full frame captures that can still show fine feather detail. Guy with fancy lens with built in TC is stuck at 560mm, flamingos smaller in frame, composition cannot be matched without post-crops, final image has less detail.
Guy with fancy built in TC flicks the switch and removes TC, then fits conventional TC on at the same time as the other guy.
and of course scenario 5)
Zebra are up close to landrover so both guys shoot 200-400 naked (although wearing clothes). Out of no where a leopard is spotted in distance, landrover cannot approach closely for some reason. Canon guy flicks some switches and has a 560mm lens. Guy without built in TC, has to take lens off and quickly add TC. This takes longer and under some circumstances could cost the shot, although changing TCs is much like changing small prime lenses in a wedding- something Raymond I am sure you are quite happy in doing otherwise you would always use a 24-70! (hangign TCs is tually normally very easy because the lens is supported by the tripod, you just unlock the camera leaving the heavy lens supported, flick on the TC like a small lightporime, and then re-attach camera+TC.
In the real world shooting in a safari vehicle with a tripod is a massive pain... How about that situation where the animal jumps out and the driver doesn't have the time to turn the engine off, all you get is blurry vision from the vibration of the engine. Gotta say next time I go on safari I'm not going to take a beanbag, they are more of a pain that handholding. And then there is the walking safaris, carrying a large lens attached to a tripod when you're stalking through a bush should be funny to watch!
Just for fun there is someone using D.P.'s patent-pending super-TC module whoe combines the speed of the Canon's guys flick of a switch transition with the Nikon guys 2.0xTC and gains the 800mm needed to capture the FF shot of the leopard.
And for completeness, there was also a Guy with an 400mm (or 600mm, whatever) prime lens who is constantly adding and removing the TCs. When that lion get really close he cannot simply zoom out like the 200-400 owners, but is forced to remove the TC (and likely change lenses). If this prime lens had the ability swap between different TC settings then it would be much mere flexible. The reality is the guy with the fixed prime will either decide on a different composition (e.g. head and should portrait of the lion) or simply swap to his second camera with a 70-200 mounted.