I guess there is a little mixup here.
You can't compare Calibre to Bookdesigner or the Feedbook interface. Calibre is essentially an ebook libary with viewer and converter, Bookdesigner relies heavily on the editing side of things whereas the feedbook interface is just that, an as easy as possible interface. All three are great applications and do their work beautifully. I am just grateful for these tools and can see me using all three for special purposes