All canon EF lenses are guaranteed to work with EOS bodies.
Sigma can't claim that, that's what the dock is for. £40 dock and saves them a lot of man hours to update lenses people sent in to make their old lens work with new canon bodies.
I do agree it is useful but once it's set up and the lens works. That'll be that. I don't see it as a game changer, a century of photography went by without it and another century will too.
Of course, if your lens can utilise it then for the money is silly not to buy, if anything, it will make sure it works with your new body.
Did you read up on the dock? The hints were there in the video you posted as well.
The dock allows you to do way more than just update firmware.
- Focus speed.
- Focus range.
- Focus mode.
- Image stabiliser modes.
- Distance based micro-adjust (including zoom ranges for non primes).
Of course these all depend on the type of lens being used (Art, Contemporary or Sport). All features won't be available on Art, for example which is a shame.
But, and this is a big if, since the dock uses a standard connection and a standard driver model, just like Lacie's BlueEyePro colorimeter working with 3rd party software to get alternative features I do wonder if community based software will come out to enable more tweaks down the line. Bit like Magic Lantern with Canon cameras currently.