I was a program manager for Microsoft Word from 29 to 211 and one of the areas I worked on was the .docx OOXML format. I can't speak for the other file formats only for Word. There are a couple of big reasons why the OOXML was beneficial (or at least not harmful) to Microsoft from apetitive perspective The main factor I saw was extensibility. Word occupies a key part of many businesses' workflows. For most of its history a Word document was primarily opened by the Word client. However there's more of a need for automation of creating documents today. Let's take creating invoices for example there's no need to spin up Word client now as I can programmatically inject content directly into the XML. This wasn't possible before. So from a platform perspective this was a big win.