I don't like "closed formats". To help you understand why, I give you some links to pages from authors who think similarly. This way, I don't have to spent precious time explaining it myself.

In no particular order:

• "No Proprietary Binary Data Formats." by Sam Steingold (local copy)
• Dutch article: "Waarom vrije software en vrije formaten beter zijn voor de wereld." by Bas Wijnen (local copy)
• "Why data should be in open formats." by Bas Wijnen (PDF) (local copy (PDF))

These handle the subject "Why not to use Word(TM) format when sending e-mails (in no particular order):

• "plaintext. In praise of practical e-mail hygiene." by Martin Vermeer. (local copy)
• "Why no Word?" by Chris Mason. (local copy)
• "Don't use MS-Word as a document exchange format." by Jeffrey Goldberg (local copy)
• Article for readers who prefer Dutch (PDF). (local copy (PDF))