What's with this .docx thing?

At the office, we now have Office 2007, and Word seems to like making .docx documents as its default. Presumably .docx lets them do marvelous new things, but why not reserve it for the occasions that really call for it and let .doc stay the default? And what's with violating the 3-character extension rule, anyway?

I should note that I only use Word when absolutely forced to, and I don't have Office 2007 on my computers at home. My main problem is when someone e-mails me a .docx file, and I have to ask the sender to make me a .doc instead. I suppose I could go to the trouble of installing a converter (or, heaven forbid, Office 2007 itself), but I simply don't want to. It's not reasonable to have to accept product obsolescence and greater inconvenience without greater functionality.