Assume people collaborate using Microsoft Office documents around (probably by e-mail, but that's another discussion).
The "97-2003" formats are closed, binary formats, but well readable by applications such as OpenOffice.org, and now documented by their author (albeit under a specification that the GPL people can't yet get behind). The 2007 format is more interoperable in the sense it's XML and (mostly) documented and went through an infamous "standardization" process, but is still not widely accepted. I would rather see ODF succeed — potentially with patent-free extensions from Microsoft to allow it to support all the features of Microsoft Office — as the 'standard XML document format'.
Now we've got all that aside, I prefer .doc to .docx, and it's nothing to do with any of that - it's purely pragmatic, with respect to users of previous versions of Office.
I believe that in the team I work in, maybe 2/3 are on Office 2003 and the other 1/3 are on Office 2007. Our customers, being that we deal with very large companies, are overwhelmingly all still on Office 2003. (Personally, I'd rather send a PDF to a customer than a DOC, but that's not a decision I can make company-wide.)
I have the converter pack installed, which makes my Office 2003 installation compatible with Office 2007 documents (though it does prompt me and say that some features may be lost in translation). I can't assume that everyone does, however, so OOXML files inconvenience anyone who gets sent these documents that does not have the converter installed. The argument could also be made that PDF files inconvenience people without a PDF reader: everyone just downloads one, so what?
By way of opinions placed in the comments, should I be encouraging colleagues who send me documents in Office 2007 format, to enable saving by default in Office 2003 format?