I use some of the free themes on WordPress and although you may disagree with me when I say that switching themes is a breeze, I do think that when themes try to become plugins, that's when we lose the modularity that WordPress was meant to provide. There are certain instances when we can think of certain features to be a part of a theme, such as providing an author profile page, but at times it can be something that clearly ought to be a separate plugin that the theme author bundles to create value or that premium feel. Think of it as bundling a web browser with the operating system and most of you will know what I am hinting at.