Some of us pay more attention to aspect ratio than others. Some prefer to shoot at the camera's native aspect ratio and crop later while others prefer to compose in the final aspect ratio. At least that is what we have been telling ourselves since the dSLRs we're used to only have one option :)
One of the things I love about my Panasonic GH2 is the ability to view and compose in the final aspect ratio, including square! [An interesting side note is that I occasionally find myself still rotating the camera into 'portrait' orientation when I'm shooting in square format - which says something about how much my years of shooting 3:2 aspect has influenced my habits...]
Whatever your take on the subject, with a dSLR it just isn't an option to compose in your desired aspect ratio using an optical finder. My Canon 7D, for example, doesn't offer an option to do this even in live view where it could, nor does it offer framing in its dynamic LCD screen in the optical finder. The GH2 (and GH1 before it) offer a unique (thus far, anyways) feature on their sensor in that it is larger than the image circle in the horizontal direction, thus giving you more physical sensor areas when you go to a wider format such as 3:2 or 16:9. Thus you actually get more horizontal field of view, as well as locking in the aspect ratio choice. The point here is that this isn't a case where you can take it in 16:9 but still use the full image later - you actually lock it into the RAW file and discard the top and bottom (which would not have covered the whole width anyways).
For me, it turns out that I like composing and shooting in a slightly more square format than 3:2. 4:3 is more my style, and thus I never shoot in 3:2 with the GH2. I do go to 16:9 where I'm getting more width and less blue sky, or to square. But 3:2 simply isn't my preference. So the question I wondered was if you compared an APS-C camera to MFT in 4:3, what is the actual difference in area? My guess was that they were a lot closer than the typical stated 50% difference we're used to hearing.
A quick bit of spreadsheet math, and here are the numbers:
MFT: 13mm x 17.3mm, total area is 225 mm2.
Canon APS-C: 14.9mm x 19.9mm, total area is 296 mm2 (normally the full width is 22.3mm)
and just for fun, here is the new Canon GX1:
18.7mm x 14mm, total area is 261.8mm2 (a case in point, DPReview says it is 20% smaller than its APS-C cousins ignoring aspect ratio - using 4:3 on both would bring it to more like a 12% difference)
There is a nice image of this http://2.s.img-dpreview.com/reviews/canong1x/images/Sensorsizes.png?v=1440.
Thus, if you are cropping to 4:3 or 'squarer', the sensor size area difference is only 1.3x or less. The height difference is a mere 15%. I think it is safe to say that most discussions about the sensor size differences ignore the discussion on aspect ratio, to the detriment of MFT. Of course, if you are using Olympus or any of the non GH series Panasonic bodies you don't get the slight bump in resolution when you go to 3:2 or 16:9, but you still see the 4:3 versus 3:2 advantage - unless of course you prefer to shoot in 3:2 :) If you do actually shoot 3:2 as a preference, then the argument goes the other way!
If you look at typical portrait sizes, like an 8x10, or 16x20, then the height of the sensor is the limiting side, and the extra area of a 3:2 sensor is wasted. Worse, you typically have to compose with the extra space in the viewfinder, too.
MFT is only as good as it really is, and discovering that the sensor area difference is smaller than you thought doesn't mean the image you weren't happy with last week is any better - but when you read about people talking about the 'huge' sensor size difference, take it with a grain of salt... More and more, MFT is good enough for me - certainly for my personal photos!
How does this relate to stock? Well, 4:3 gives you a bigger overall thumbnail on Dreamstime :) I'm not sure how aspect ratio helps or hurts for stock - I think buyers are savvy enough to know they can crop, and usually appreciate a bit of extra room to crop either way.
Here are a couple of shots of the kids on my personal blog taken in 4:3 ratio: http://mftadventures.blogspot.com/2012/04/mft-and-kids-portraits-action.html. What is your preferred aspect ratio?