Four reasons not to use proprietary formats

Proprietary formats can be used locally without risk by any user for his own personal use or by a group of users who have formerly decided to use a specific format and a specific software for cooperating on a given project. On the other hand, sharing files is a social act, whose effects concern not only the user itself, but also the set of all possible users of these files. Sharing files requires converting the original format into a format appropriate for file exchange. Open formats are by definition exchange formats: they insure accessibility, interoperability, and perennity to the data.
When exchanging files in proprietary formats, you contribute to the propagation and enforcement of undesired practices.