Although I sort of agree with you from a data management point of view, I chose to have a separate caption field as that's the way the image block manages it and I feel that it's not obvious enough on how to edit a file's description field. At least, I know most of my clients would have trouble finding where to edit the description for a file.

There are also cases where you might want to have a shorter or different caption, but not want to edit the 'main' description for an image.

I appreciate your feedback though, it definitely made me consider some options.