It would save me a lot of fiddling around if I could set the Caption for an image in a Lightbox gallery or a Carousel to be the alt text for the image. Apart from anything else, it would encourage me to fill out the alt text!
I see this as a UI event in the sense that having called for the Alt text to be the source for the Caption the text would be simply copied from one field to the other. If done this way, any subsequent editing of either would not necessarily affect the other, which would be fine by me, although it might cause confusion. To get around such confusion, in the UI if this option were chosen, merge the fields into one that could be separated again if the designer changes his/her mind.
Dave