I've had a number of customers ask me lately for the reason why Microsoft have chosen to use the new Open XML file format in the Office Applications instead of other existing formats.
Instead of reinventing the wheel and trying to explain the reasons myself (badly) I can recommend Brian's blog for a very comprehensive explanation as to the reasons behind the new formats.
Brian Jones is a program manager at Microsoft. He's been working on the XML functionality and file formats in Office mainly focusing on XML in Office and the Open XML File Formats coming in the 2007 Microsoft Office system.
To view Brian's blog click here