OK lets make the comparison simpler. The new 16:9 vs the old 12:9. Keeping the the height (vertical or 9) gets you 1/3 more screen real-estate and yes I would go for that but that's me. With 33% more display area I could certainly show more relevant data. It increases the length (horizontal) by 2.6" still within the DL footprint. Check your post something doesn't add up (what's vertical).
As far as vertical, I'm referring to when the iPad is held with the home button at the bottom in "portrait mode."
Pixel resolution is not in the picture on aspect ratio. The production of 16:9 already outstrips 4:3 by leaps and bounds and Apple will have no choice anyway. Have you tried to find a 4:3 monitor lately? They're pricey if you can find them. Most phones including the iPhone are close to 16:9 as well. Let's face it it's a 16:9 world today.
I'm well aware that pixel resolution and aspect ratio are independent items. I've been in TV/film post production & broadcast industry for over 25 years and currently am an engineer at a well respected pro video hardware manufacturer. What I was referencing is that with the high pixel density of the iPad's Retina display, you are not degrading the resolution of 1920x1080 the native HD raster when you view HD material on said iPad where the top and bottom of the image is being masked with black. So the fact that the iPad is 4:3 doesn't really affect the quality of HD video viewing and gives additional real estate for other apps as well as basic OS stuff. I realize you're posting in the context of the DL, but there are many other uses of the iPad outside of the DL.
I hear you about 4:3 displays in general though. Unfortunately the Avid Venue I work on still uses 4:3 aspect for it's graphics output. I had to find an old NEC display to use with it when I got it or I'd have to have a stretched display which I loath. In that sense, since Avid's uses VNC for remote control of the desk, I'm grateful the iPad is 4:3 when I'm controlling the desk remotely. When I'm using my iPhone 5 with the 16:9 aspect ratio to control the Venue, it sucks having a 4:3 VNC screen inside the 16:9 display. Even the new Avid SL3 desk uses 4:3 output which is silly. I guess Apple isn't the only one behind the times as far as your view on aspect ratios