[HTTPS-Everywhere] Downgrade rules vs exclusions

[Jacob S Hoffman-Andrews]

In trivial-validate.py we get a lot of warnings about rules that 
redirect to HTTP. This seems like a normal thing, since some parts 
of sites may not support HTTPS. But there's an alternate mechanism, 
exclusions. Can anyone fill me in on why both mechanisms exist? 
Would it be correct to reformulate all downgrade rules as exclusions 
instead? Two examples below.

Fourmilab:
https://gitweb.torproject.org/https-everywhere.git/blob/HEAD:/src/chrome/content/rules/Fourmilab.xml#l32
         <!--    Without the following downgrade rule, images requested via the URL
                 starting with http://www.fourmilab.com.ch/cgi-bin/uncgi/Earth? or
                 http://www.fourmilab.com.ch/cgi-bin/Earth? may not load completely.
                 (February 26, 2013.)
                                         -->
         <rule from="^https://(?:www\.)?fourmilab\.ch/cgi-bin/(?:uncgi/)?Earth\?"
                 to="http://www.fourmilab.ch/cgi-bin/Earth?" downgrade="1" />

Zipcar:
https://gitweb.torproject.org/https-everywhere.git/blob/HEAD:/src/chrome/content/rules/Zipcar.xml
         <target host="*.zipcar.com" />
                 <exclusion pattern="http://members\.zipcar\.com/(?!apply|regist)" />