Well they do :) here's a regex that will match any file if it does not contain "Copyright" (can be quite useful, that :p).
(?s)\A((?!Copyright).)*\Z
(?s)\A((?!Copyright).)*\Z
That left you with no way to compare your files, save for renaming them to another extension. Well fear no more! As of 0.9M3, EMF Compare now displays a pretty error message when something goes wrong while loading the models.
Apart from frustrating you with this all too often seen red icon, this dialog provides you with the option to compare the files as text. Oh goodie!
Well of course, xmi isn't really a human-friendly language ... but at least this is way better than nothing to spot that stupid typo and make the model useable again :).