"…For Competitors: The move to published, royalty free XML makes Microsoft less—not more, as many other analysts have asserted—vulnerable to open source competition from such sources as Open Office and Sun, with its StarOffice suite. Open source systems appeal was in the low (if not nominally free) license cost, accessibility, and ease of integration across enterprise applications. For most enterprises, these factors had to be weighed against the possible disruption of business caused by introducing a new, less refined office productivity suite to massive numbers of long-accustomed Office users. By using Open Office XML, Microsoft will effectively take accessibility and interoperability out of this equation."
Although this holds true, this move will reveal a weak spot on Microsoft's end, which will be the ease of migrating documents to other formats.
Today a strong lock-in holds many users to keep on using MS-Office except for reasons of product excellence and that is proprietary file formats. Opening these formats via XML will ease migration to other formats and will open up "fair" competition for other MS-Office alternatives.