In general, plain text is great because it's explicit. In contrast, word processors store some odd chain of objects. The steps to rebuild a document from scratch that leads to the same representation are not totally obvious. Sometimes it's a bit stochastic even!
XML formats improved the situation, but still plain text wins IMHO. With plain text you can have a toolchain where components for version control, edition and transformation can be replaced seamlessly. New stuff can be introduced at will. In e.g. Word you're mostly locked-in, or at least it's far from trivial to break out.