I see this as an example of Conway's Law - you have one team thinking about a document, and one team thinking about turtle-graphics. Of course, they will create something where changes in the document structure don't affect the turtle-graphics-people, and changes in the turtle graphic part don't affect the document people.

My favourite abuse of XML is the MOSMIX format (pdf), which is a KML dialect with fixed-width additional weather data. The justifications to me are that the fixed-width data already existed from older formats and that wrapping each column in additional XML would have bloated the (uncompressed) file size a lot. But still, it's a funny abuse:

<kml:Placemark> <!--Beginn einer Vorhersage für einen Punkt -->
<kml:name>01025</kml:name> <!--Stations-ID -->
<kml:description>TROMSOE</kml:description> <!--Stations Name -->
<kml:ExtendedData> <!--Hier beginnt die Vorhersage der einzelnen Groes
+sen -->
<dwd:Forecast dwd:elementName="TTT"><dwd:value> 272.35 272.45 272.55 2
+72.85</dwd:value></dwd:Forecast> <!--Eine Vorhersagegroesse-->
[download]

Other than the format, the data feed is nicely stable and works with Weather::MOSMIX for six years without changes already.