>That said I still don't get why anyone would benefit from >using that solution and letting the camera throw the pixels >away at the point of capture to get the "crop >factor" for more reach. > >Surely a custom crop (or aspect ratio constrained) would >always be better in post
True and I agree, but there are thoses situations that you might want not want to fill up your cards buffer with extra pixles. Like when I shoot sports. I know I'm going to crop anyway so If you frame correctly you can avoid buffer and or cf card being filled up. I would choose to shoot in dx and save the buffer in that situation.
I think compositions is over rated to a point. Granted a little wiggle room is good to have. Some of my best shots have come from prime lenes where I thought I may have shot to tight. If you can't compose your shot and your shooting zoom then somethings not right. Just my two cents.
Now if I had the abality to use a longer lens I would do so and If I wanted to save buffer I might reduce the file size of the image in camera. That would be an expermiment as that is not part for my work flow.