While longevity may be important, I think there are other more interesting advantages of plaintext, namely that I can edit in my editor of choice, I can use a host of existing tools, such as sed, grep, etc, to process it, and it plays well with standard source code management systems.
If you ever wrote a paper with another person, you will absolutely hate Word (and other binary formats). It makes working on the paper a pain, since only one person can work on it at the same time.
The ability to split a document in multiple source files and having SVN/git/etc makes it really easy to work on the document at the same time.
