Assumptions: We don't care about IE6, and Noscript users.
Lets pretend we have the following design concept: All your pages are HTML/CSS that create the ascetics, layout, colours, general design related things. Lets pretend this basic code below is that:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
    <head>
        <link href="/example.css" rel="stylesheet" type="text/css"/>
        <script src="example.js" type="text/javascript"></script>
    <head>
    <body>
        <div class="left">
        </div>
        <div class="mid">
        </div>
        <div class="right">
        </div>
    </body>
</html>
Which in theory should produce, with the right CSS, three vertical columns on the web page.
Now, here's the root of the question, what are the serious advantages and/or disadvantages of loading the content of these columns (lets assume they are all indeed dynamic content, not static) via AJAX requests, or have the content pre-set with a scripting language?
So for instance, we would have, in the AJAX example, lets asume jquery is used on-load:
//Multiple http requests
$("body > div.left").load("./script.php?content=news");
$("body > div.right").load("./script.php?content=blogs");
$("body > div.mid").load("./script.php?content=links");
OR---
//Single http request
$.ajax({
    url: './script.php?content=news|blogs|links',
    method: 'json',
    type: 'text',
    success: function (data) {
        $("body > div.left").html(data.news);
        $("body > div.right").html(data.blogs);
        $("body > div.mid").html(data.links);
    }
})
Verses doing this:
    <body>
        <div class="left">
            <?php
                echo function_returning_news();
            ?>
        </div>
        <div class="mid">
            <?php
                echo function_returning_blogs();
            ?>
        </div>
        <div class="right">
            <?php
                echo function_returning_links();
            ?>
        </div>
    </body>
I'm personally thinking right now that doing static HTML pages is a better method, my reasoning is:
I've separated my data, logic, and presentation (ie, "MVC") code. I can make changes to one without others. Browser caches mean I'm just getting server load mostly for the content, not the presentation wrapped around it. I could turn my "script.php" into a more robust API for the website.
But I'm not certain or clear that these are legitimately good reasons, and I'm not confidently aware of other issues that could happen, so I would like to know the pros-and-cons, so to speak.