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They moved to new extensions to represent the completely new format, and moved to an XML driven system. This should (in theory) make their files a bit more compatible with other non MS products.
I agree with you in theory. The only problem is, I know their track record. So we have historical evidence to indicate that the Office 2007 file formats are just another rung in the same old ladder; the files are not going to be any more forward- or backwards-compatible than any previous version was.
It may make them easier to parse by third party products, but that won't help someone trying to open an Office 2013 file in Office 2007. They'll still run into the same problem, mark my words.
I believe that MS does all this on purpose, as yet another planned-obsolsesence to make people upgrade the office suite.