Yeah, we get quite a few requests for that but there are two big problems:
1) We still need to display toolbars at the top/bottom for the reader, and in a paginated text we can't simply scroll the selected range into view if it's covered by a toolbar, so we have to preserve those margins even if we don't put any controls there in order to make sure that you can see the entire page with the reader visible.
2) Because tapping on characters is how you look them up, the usual reader-app trick of showing the interface when you tap anywhere on the screen doesn't work for us, so we don't have a good way to hide/show controls. (we could of course invent our own way, but if it wasn't a standard / obvious one then a lot of people would get confused and annoyed when the controls disappeared on them)
However, if you turn off 'paginate' in the reader settings (gear icon in the top toolbar), that will get rid of the bottom controls and let text go all the way to the bottom of the screen. (this is not currently supported in EPUB, but it will be in 4.0)