I see the point for cases where multiple folks are editing the document and everyone is in agreement on this as the approach that will be used.
In both cases, the changes made are clearly tracked.
My concern again is for the situation where someone modifies a document (inadvertently or maliciously) and passes off the changes (inadvertently or maliciously) as still belonging to the original author. This is far easier to do with a Word file than with a PDF file. Granted, as needed, we can check behind, for example by looking at the date+time modified stamps. In essence though, submitting a Word file as a document of record says to me that you accept that it can be changed even without your permission, the changed file can be re-distributed or evaluated as though it is still your document, and you may have to bear the full consequences to rectify any damages that might follow (inadvertently or maliciously) from your record being modified and passed off as yours.
To make a specific case (here in counter to your note about submitting Word files at your university) , I will never accept a Word document from my students as their final report. I insist on PDF files. Even though I hold myself to a standard of integrity, the chances that I might muck up something inadvertently while I am grading their document of final record are too high. PDF editing tools are sophisticated enough now that I have no reason to use Word files “because they are easier to grade” (or whatever other reason might be given to request a Word file).
“Trust everybody, but cut the cards (yourself)” comes to mind here.