You should be able to use it for other RPCs. Example xml-rpc post for user.update shows an example of a user.update method call.
<?xml version="1.0"?>
<methodCall>
<methodName>user.update</methodName>
<params>
<param>
<struct>
<member>
<name>uid</name>
<value><int>38</int></value>
</member>
</struct>
<struct>
<member>
<name>name</name>
<value><string>diana.ruiz12</string></value>
</member>
<member>
<name>mail</name>
<value><string>[email protected]</string></value>
</member>
<member>
<name>pass</name>
<value><string>candy9223</string></value>
</member>
</struct>
</param>
</params>
</methodCall>
It doesn't look like a widely used feature, and it will be moved out of core in Drupal 8.
See also Services Tutorials and Examples.