DefaultKeyBinding.dict - old-style NeXT format vs XML plist format
I don't think you'll have any issues using the older format.  All the examples I've found use the old NeXT notation.  Although I can't explain why the XML format didn't work, there is a workaround for this particular keyboard shortcut which would mean you can leave your existing DefaultKeyBinding.dict untouched.  

There is an application called KeyRemap4MacBook which is free to download and you can see from the screenshot below, you could remap the ⌘+⌫ keyboard shortcut without converting the rest of your DefaultKeyBinding.dict file and this method is not limited to Cocoa applications.