php includes only work if the other page is a php page as well. There really is no reason for him to use php unless he starts using dynamic content with data (such as a forum or a user base).
Money, let me ask you a question, if all you did to a page was include an iframe what would be the use of the iframe if nothing else was on the page? Just go directly to whatever page you've targeted.
Basically, you use iframes when there is a small area that you wish to change the page of, such as a content frame or something as such. You could have the main page always stay the same (runs faster if you put the graphics on that page too) and then some frame in the middle (iframe or normal frame, same thing, just one can be moved around) to have the content of the page.
It's also not that hard to make iframes look decent in all the browsers, so I don't know what you're all going on about there. All you do is define it for them all. (mozilla allows right, left, top, and bottom alignment, IE doesn't. But IE DOES allow widths and heights. Easy thing to do is put both in the styles and tweak it so it works in both browsers)
I'm not sure what else to explain, I don't know where you got the idea that an iframe replaced the body tag. Maybe you're confusing it with normal frames?
Frames in general are just good for navigational purposes. However, it gets a little tricky when using popups as well. I made a site for a project about a year ago (doesn't work in IE very well, but that's because I didn't go through and tweak it and we targeted a specific browser and went to work) and I incorporated all these things together.
And iframes are just about the only good web-development item to come out of Microsoft, thus it being adopted by W3C.
The IFRAME element allows authors to insert a frame within a block of text. Inserting an inline frame within a section of text is much like inserting an object via the OBJECT element: they both allow you to insert an HTML document in the middle of another, they may both be aligned with surrounding text, etc.
You should really check out thier site, it has everything you'd want to know and how to use it.
http://www.w3.org/
Blame it on Microsoft, God does.