It is there so that Python implementations (other than cPython) that do not have ElementTree accelerator modules can fall back on the pure python version.

You can import the pure python version in cPython by blocking the import of the C accelerator:

   import sys
   sys.modules['_elementtree'] = None
   import xml.etree.ElementTree

I'll leave this open to see what the xml experts think about the custom parser idea.