For some time I've used the +++siblings for front pages, where font size and formatting varies from line to line. I've also found it handy on several other occasions.
I've been sorely tempted to drop the <p> tag altogether, and just use formatted <div>'s for paragraphs instead. It's so often I want to include some kind of block-style element inside a paragraph, only to yet again experience the epub/html validator sending me dirty looks...