I generally want to keep captions unobtrusive, and blocked together. So I want more than just one line of text, I want space for them on far more than the tiny number of layouts where you provide them, and I want control over typeface, including the ability to put some text in bold and/or italic. I also want to overlay captions over the photo rather than put them in a block at the side. And a choice of simple black vs white for backgrounds is very restrictive.