Asking this question here is begging for an abundance of the Apple-endorsed answer. Apple has deemed 4:3 as the one and proper screen ratio... so it is. If Apple then decided that 16:9 was the new one and proper screen ratio and you posted this question a month or so after that change, then most of these posts touting 4:3 as the best aspect ratio for tablets would be doing the same for the 16:9 version.
See the before and after comments about Apple-endorsed choices such as:
- PowerPC chips vs. Intel
- No facetime in iPad1 vs. facetime in iPad2
- 720p Max in
TV1 & 2 vs. 1080p in
TV3
- old iPhone screen format vs. the new taller (but not wider) one
Etc. In short, the way Apple appears to endorse is THE one, correct way... until Apple shifts it's endorsement... and then the new way is THE one, correct way. We'll spin and spin the rationalizations to support whatever Apple endorses and then flip with Apple when Apple flips. Then, we'll spin the new way as if we always thought it was THE way.
Since this is an iPad section, I point anyone doubting this to the iPad 1 launch posts where there was fierce defense of Apple's choice to NOT include a front-facing camera in the first generation. "How would I hold it steady?", "Why would Skype people want to look up my nose", "It makes no sense to include a camera for video chatting in this thing", etc. Then, one year later, Apple rolls out facetime and some of the very same people are "Shut up and take my money", "I'm already in line to buy one", etc touting facetime as if it was the second coming.
All that said, I own a new Mini and am quite happy with it (and it's screen "as is"). I could easily make the same case of how 4:3 is a great choice. However, 16:9 is mainstream and Apple going that way would probably result in a lower bill of costs (which might help get prices down a bit across the line). 16:9 could yield a thinner device that still had a lot of screen space. Virtual keyboard could get larger or the spacing of virtual keys could widen a bit more. Wide-screen movies & TV shows would fill more of that screen rather than being significantly pinched on the 4:3 (yes, you can zoom to fill but then you are chopping off a good amount of the left & right). More of longer web pages could be on screen at the same time in portrait mode (less scrolling). Apps that favor 4:3 output such as page-oriented apps could shift controls to the left or right of the page so that the page could go edge-to-edge (instead of having nav options squeezed in above & below a page); such apps might also be able to do some side-by-side page presentations (2 up) which might look just fine on a "retina" 16:9. And so on.
Of course, all that makes no sense/"is stupid" because Apple has decided that 4:3 is THE correct ratio for a tablet. But check back after Apple would change it's mind and the chorus will then sing the praises of the new change (see iPhone 5 stretch when it was a rumor: "stupid", "Apple doesn't want Android-like fragmentation", "Apple doesn’t want another screen size to make things more difficult for developers" and after: "this is way better than my old iPhone", "it's so much nicer to have the extra screen space for ___________", etc.).