Based on my experience a positive side for iframe are when calling third party codes, that may involve calling a javascript that calls a has a Document.write(); command. As you may know, these commands cannot be called asynchronously due to how it is parsed (DOM Parser etc). An example of this is http://sourceforge.net/projects/phpadsnew/ I've made use of iframes to help speed up our site as there were multiple calls to phpadsnews and the site was waiting for the response before proceeding to render different parts of the page. with an iframe I was able to allow the site to render other parts of the page and still call the Document.write() command of phpads asynchronously. Preventing and js locking.
