Mmmm, that's quite a significant difference. I hadn't come across such a difference between a JPG and an "as shot" RAW before. I wonder what exactly is coming into play there.
Even if you forget this visible difference to the two "as shot" versions, the editing or correcting capabilities of the RAW format is second to none. What I really love is the seemingly very wide dynamic range - when you fire up Lightroom and adjust the exposure, for example, you expect there to be very apparent limits to that kind of adjustment (ie clipping, which would occur in flat images like JPG), but it just keeps showing nice gradiations throughout the tonal range! It's the best!
To get back to your original query though; As far as I can see it, there's no reason why you must
shoot in RAW for ultra high quality output such as stock, but it can certainly help. As has been stated, it simply allows for much greater flexibility after the shutter has been pressed.
With that said, if you get exposure and white balance right, I don't see why you can't produce a top notch result with a high quality JPEG.. ?
Buuuuut.. When I get my D300, I think I'll shoot in RAW+JPEG no matter what. It just means more hard disks, and a damn good archival regime.