Most the tools we have for WYSIWYG document creation lacks determinism and portability. For the latter, one could argue that there are open formats, but still, there should be a program that can interpret the format; whereas with plain text, such a need is void, as it is possible to view the document on any decent system. For the former, the argument might be that the modern word processors provide the facilities to deterministically lay out a document through the user interfaces they sport, but then because the formats they save are either are endemic to themselves or badly supported in other software* this feature is not of much use.

I can open a Markdown/ReST/Textile/... file with any text editor, including Notepad, Vim, Emacs etc., and also view it through more or less programs, or just cat them. I can pass them through head or tail; search them with common utilities and even edit them with some others. I am not bound to any programs in order to edit my program. If, on a computer I have to use, there is no Word, or Writer or Pages, I can still edit/read the document. I can read it online, via a browser. I can use programs that are decades old, and I also will be able to read the file decades later. When I send the file to someone else, I can be sure that they will be able to read it. Any usable operating system has a text editor bundled. This level of portability is just a dream for WYSIWYG editors. For these advantaged, I happily trade editing convenience off.

* Last summer, my cousins needed to use my computer for editing a docx document that was important for their undergraduate education. I was running Ubuntu OS at the time, so I told them to use the LibreOffice's word processor. The experience was bad; the document did not render properly, editing was problematic. This is the only case I can provide as an example to support my argument, as it has been multiple years since I used a word processor program.