For me iframes are pure evil (well, maybe not so pure). They seems to make a lot of troubles. Yes, your whole site will load once and then you can just load a single pages. But people invented AJAX for this purpose.
One of the biggest issues I found with iframe
was that I couldn't paste a link to one of the subpages, because the URL never changed (yes, I know there is a workaround for this). Second thing, web search engines may have problems to index that sites correctly.
Sometimes the accessibility of this sites are worse and some browser can even display them improperly.
There are better ways to design layout without (i)frames. Everyday I can see some one asking at SO questions, like "How to access iframe with jQuery?".
So what are benefits of iframes? What reason can it be to still use them? I just would like to know why :)
(since it is not a real question, it's a CW)