So the overhead of semantic markup of the whole content (instead of just marking words as bold) might be worth it.
I dont think either of them have to be turing complete though.
I recently started using it because Lyx does not work on my ARM based Linux tablet and its not too bad when used with a GUI editor.
It is a lot more powerful than other things I use (Markdown, Sphinx) and a lot better for version tracking and multiple output formats than word processors.
XML is a good start to describing structured text.
If you're compiling a report for a college group project (or the workplace equivalent of that) you might well need things like equations and tables and suchlike.
For example if you've got a table of performance results with the best performer in each row highlighted in bold, a fact the text references - then separating content and formatting doesn't really make that much sense.
You can tag the text as "important": see <strong> vs. <b> in HTML.