Fair points seancron. Wirify is already skipping quite a few HTML elements in its processing, however it certainly works better on some sites than others. I've found that proper 'semantic markup' also gives better results than old school table-based layouts for instance.
One of the earlier prototypes let you control the 'level of detail' in the wireframe: http://www.youtube.com/watch?v=wuKJVZRDjRo We've decided to ditch this idea for the time being though due to the extra complexity.
Certainly if someone was to 'rip' an existing website design wouldn't they just grab the HTML+CSS directly? Wirify is definitely not intended, nor useful, for that.
In some cases. In others, you might be starting with a simplified wireframe (and these remove identifying brands, colours, etc) or you would screen capture and use it as a layout base in PhotoShop.