They should add a couple more speakers and have 'L' and 'R' follow the orientation.
Cool idea. Let me jump on that and jump the shark at the same time.
1. Let's your idea adaptive stereo if it doesn't already have a name. Four speakers that act as stereo pairs and are sensitive to orientation.
2. Let's throw in quadraphonic because that's a good logical next step. You'd essentially have (most of) the hardware to do it with #1. But here, when the data is available, each speaker gets its own sound to drive. Some older music could be found that was mixed for four speakers. Otherwise it could open up new ways to remix music. But this would be particular useful to app developers who could really separate sound output in interesting and immersive ways (we'd get a lot of "spinning" effects, and 3D games with well mixed sound locations).
3. Let's float the idea of being able to use speakers as mics and vice versa. Because of the iPad platform's size, this could be useful to repurpose hardware that's not being used. I'm not sure if anybody does this, though it's technically possible. There may be perfectly sound reasons why that's the case.
Let's throw in a couple middle ground options:
4. Mono speaker on top and bottom, but when held in landscape they function in stereo as left/right. This is is essentially your idea but 2 speakers instead of four, and we're doing away with the poorly separated stereo that we'd have in portrait mode.
5. Mono speaker on top, stereo on bottom. So you have crappy stereo in portrait, and better stereo in landscape. Only three speakers to drive total.
Some of these gets into "retina" territory, of Apple creating their own standard, justifying it with pseudoscience, and slapping a catchy name on it. Let's call them "Cocclea" audio, though it's quite unappetizing.
Okay, now back to reality. First, does doing any of these presents a very minimal change to the sound profile. We're dealing with tiny, tinny drivers encircling a waif of a device. Quadrophonic speakers in the 80s were big things that filled rooms with diverse, separated sound. A far cry from what any tablet is kicking out today.
Second, bigger problems need to be navigated here. There are a few ideas listed here, but Apple is limited by A) size/weight, B) cost/competition, and C) fragmentation concernts (they strive to present an unfragmented device ecosystem to allow for coherent developer and content/media support). Apple has shown excellent sensitivity to all of these, which need to be balanced.
I could ramble on here, but I would hope that apple would continue to balance these concerns. As much a single device bullet point could be nice, it disrupts the ecosystem lineup. So if Cocclea sound is coming, I'd want it deployed uniformly across iOS (at least iPad), and ideally Macs and headphones.
Short of that, adaptive stereo could be cool and is already supported by developers and content providers. But again, I want a coherent lineup: even though the iPad mini speakers are positioned on the wrong side, why does the mini get a bullet point that is denied the iPad 4?