I guess it really is, "To each his own." For me, I'm actually in the process of changing updating all of my HTML as well. But the reason has nothing to do with file size or servers for that matter (as things used to be 10-11 years ago). Rather, it all has to do with standards.

Most people who are not web developers, per say, think that HTML is HTML. However, in the web development world, this is not the case. The current HTML standard is XHTML 1.0/1.1, which is NOT the same as HTML 4.01, which was the previous standard. In the XHTML specification, most of the HTML tags are written differently and many of the previous HTML tags that are widely used in 4.01 are depreciated in XHTML. For example:

HTML 4.01 Image Tag:
Code:
XHTML 1.0/1.1 Image Tag:
Code:
Example of depreciated tags:

HTML 4.01 Font Tag:
Code:
<font face="verdana" size="2">
XHTML 1.0/1.1 "Font" Tag:
Code:
<span style="font-family: Verdana; font-size: 14px">
To be honest, following current web standards actually leads to larger files. However, following current web standards leads to fewer browser incompatibilities, which is the goal of XHTML. With browsers from all fronts moving towards better support for things such as XHTML, Ajax, etc, it will soon be upon us where having a site written in older standards will either be slower or not look the same. Even today I'm starting to see this happen (in which some websites look one way in one browser and different in another) more often.