Maybe he doesn't want to? The thing about Aspect ratio is that it should be the directors choice, not a limitation of the hardware choice.
Although, from sebas's first post, it seems that he was worried that he was losing part of the picture, due to the black bars. But, in fact, cropping to another aspect ratio would actually cause him to "lose" the top and bottom of a 3:2 image.
I didn't really want to bring up framing, and how it's best to know what aspect ratio you want your film to be in from the beginning - so you can mask (or crop) bits of the frame out later to your desire. It may be necessary to mention, however. If he's framing to 3:2 when shooting, cropping to 16:9 or 2.35:1 won't particularly look good, as this will change the framing.
If you're used to shooting at 3:2, and have framed shots really well in this ratio, why not use it? Changing to another ratio, while it is your choice, would require a change in framing. If you've already taken pictures in 3:2, you may have to reshoot, depending on how harsh the cropping is.
I usually shoot at 4:3 on my camera (although, I have occasionally used it's 3:2 setting as well), but even then, I'm not framing to 4:3... I frame things specially, so no "useful information" is cropped off when I cut the ratio down to a wider standard.
If you can't shoot anything other than 3:2, but want 16:9 (or another, wider aspect ratio) then I'd recommend you take some test frames, just to see how much will be cropped off so you can visualize this when filming. I used to put some duct tape over the top and bottom of my camera's screen to aid in this. I don't do that now, though, because I've acquired a pretty good knack for guessing just how much of the base frame will be cropped by the final export/upload.
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