Apps for Office/Office Add-in Library for Managing Office Documents
VBA/VSTO had different COM interfaces for managing PowerPoint, Word, Excel, etc. While it appears the end result is to give users access to the underlying OOXML (currently only available for Word), it will still be useful to allow users to navigate the schema of an Office document and do things like add PowerPoint Slides and Shapes without copy/pasting large XML blobs. Additionally, while OOXML support still seems far away for non-Word documents, the main components of this library are likely in use internally in creating the online versions of Word, PowerPoint and Excel.
To make the point obvious, Office.js bindings are fairly new and only really support Excel properly (named bindings are pretty useless in Word/PowerPoint, which leaves only binding to the currently selected item). This means that there is no real way to manipulate a PowerPoint slide that is not currently selected, or to add a PowerPoint Shape anywhere in the presentation.