Currently, Microsoft Office files, in various incarnations, are still the dominant format for mailing documents. Although everybody (including Microsoft) agrees these formats are not standards-based en hence (at least in theory) not future-proof, this is a situation that is not easy to change.
Try sending an ODT* file to a colleague. Chances are she will have no idea what to do with it. But since everybody knows Word, Excel and Powerpoint, the situation will remain for a while.
Here's a wild idea for improving the situation: when sending an email, most mail clients support a standard for sending both text and HTML email as alternative formats in the same mail. Perhaps a similar approach could also be applied to DOC/XLS/PPT attachments: For example: every time a user sends an ODT file, a corresponding DOC file is automatically attached as well, and vice versa.
* ODT = OpenDocument Text, an ISO standard format for text documents.