Wraps vs empty tags?

Once more, I curious about the “best practice” for something.

When in NEED of extra hooks for CSS backgrounds, lets say to build page shadows or rounded corners… or whatever presentational effect is required, why does it seem to be the preferred method to write markup/css to use wrappers elements vs empty elements, even when BOTH methods use the same number of tags?

for example…
why is it better to do this :

<div class="box_outer"> <div class="box_inner"><div class="box_left"><div class="box_right"><div class="box_content"></div></div></div>content...</div>

than to do this:

<div class="box-top"><div></div></div>
<div class="box-sides"><div>content</div></div>
<div class="box-bottom"><div></div></div>

actually the above example is still too “gray”… lets say you wanted to do a page shadow effect…

why is this considered better mark up to try to style:

<div id="shadow_outer"><div id="shadow_inner>
    <div id="page"> content... </div>
</div></div>

vs


<div id="page">
     <div id="left_shad></div>     
     <div id="right_shad></div>
     <div>content... </div>
</div>

Just curious what the forums experts take on this dilemma is, and most importantly WHY?

The additional hook elements are kept “inside” rather than being “exposed” to neighbouring layout changes…

…and for the sliding doors technique…

eek. I forgot to elaborate. I am thinking of when AP is used. T ake the second part of my question… years ago I found a technique featured in CSS Zen garden, to make a page border… that was 100% height ( or at least gave that illusion). The idea was that at the end of you page wrapper you had two divs with fixed position and height 100%… voila…100% page borders… the complain was , of course that this method used 2 blank elements. … there were more complex methods suggested which WRAPPED TWO OR MORE element around the page and used the sliding doors techniques,faux columns and such.

the sliding door tech has no advantage in the number of tags. so then I assume its because of your first reason:

The additional hook elements are kept “inside” rather than being “exposed” to neighbouring layout changes…

which would mean for a page wrap ( since there few if any layout elements…) why the mass preference to struggle with wraps, instead of using FP/AP?

http://www.csszengarden.com/?cssfile=069/069.css Yeah the design is UGLY, but the idea is clever…