Crowded markup is advantageous these days for a couple of reasons:
Anyone who wants to steal part of your code to make it their own will have a harder time doing so when all the HTML is crowded together as one instead of html with nice comments between blocks of code.
The amount of bytes required for crowding your HTML is often less than that of pretty looking HTML since spaces, comments, and the carriage return sequence take up extra bytes.
The fewer bytes your HTML is, the faster your page is downloaded and the less stress there is on the server.
What I suggest is to cram your HTML code together while still making the code run the same way. For example. Change:
<!-- Start a pretty looking box -->
<p>This is a pretty box</p>
<p>with some text</p>
Share the text
<!-- end pretty box -->
<div ID="box"><p>This is a pretty box</p><p>with some text</p><a href="http://sharer.com">Share the text</a></div>
and you'll notice fewer bytes are downloaded and the overall loading experience will be better.