Originally Posted by Jellby
That would be very device- and settings-dependent. In my device, with my settings, there's nothing at the bottom, the whole screen is available for the text. Besides, if I activate the footer, a line/rule is drawn and, without a margin, the text could clash with the line (or not, that depends on how this is actually implemented); other devices display the "status bar" with black background, which would be pretty much the same. In any case, your same argument could be use for the top margin, as I could also enable a header in my device (showing the book title and author).
This shows why it is a good idea to keep style simple and compact. Simple styles are easy to understand and modify, if a user thinks the bottom margin is too large.
But, given that the recommendation is for a very small margin, this footer would still get text on top of it. If the Reader does not push the text up when the footer is activated, then it's a bug in the reader app. Don't penalize others for a bug in your reader.