The reason we want uncompressed HDMI output is because the compression is what hurts video quality the most. With a clean HDMI output...not only uncompressed, but free of any camera overlays like the blinking recording indicator...we can attach external recording devices to the camera that allow superior compression or even no compression at all. That is still not as good as RAW format straight from the sensor, but devices are available that will probably do a little better than Canon's ALL-I mode and more will be developed in the future.
So it's a way of future-proofing the camera to a degree. The Nikon D800 offers it and that's a plus.
The other issue with the HDMI output is what resolution is available for an external monitor. Shooting video you will immediately discover that the core problem is pulling focus while you're shooting. Viewfinder attachments onto the back of the camera are one approach, another is using an external monitor. There are focus assist features on some external monitors such as "Peaking" (which outlines in-focus areas as a confirmation) or "pixel-for-pixel" which zooms to a 1:1 display ratio.
The 5DMkII didn't support those modes well at all--the resolution and proportions shrunk miserably. The 7D was a big improvement, streaming 720p, and it appears the 5DMkIII maintains full 1080p while shooting. That is a huge advance for focus pullers if true. Sadly, the 5x/10x digital zoom you use in LiveView before shooting still cannot be activated while recording to assist with focus then...so external monitoring with peaking/pixel-for-pixel is the best available approach right now.
While clean HDMI out is a nice Nikon D800 feature, I'm going to guess that the ALL-I mode's bitrate is quite adequate, and the 5d3's superior low-light performance and other improved functionality makes it retain the crown of best DSLR video system. Sure, Canon may be trying to protect it's "C" Cinema cameras a bit, but a lot of this may simply have to do with the fact it only has one Dig!c5+ to pipeline the data through and that keeps cost, weight, and heat down.
Most likely the 5D3 isn't going to be what kills your film regardless. The script...
well, thanks for explaining, rather than just foaming at the mouth like most of the others. But allow me some questions:
Why would you want an external recording device for video with the 5D3? surely you'd use a RED for something like that? the point of an SLR I always thought was portability and affordability.
As far as video output goes, it encodes in H.256 anyway, which is pretty compressed as is. If you want uncompressed HD, Canon want you to buy the C300. in terms of out, I defy anyone to tell the difference on a field monitor between h.256 720p and 1080p. Unless your monitor is HUGE.
I take your points about zooming on an external monitor. I guess I've never wanted to use one. It's preference, I suppose, but I really think "NO CLEAN HDMI OUT ZOMG!!11eleven" has become the battle cry of the "it ended up too expensive and I need to convince myself it isn't worth it" crowd.