We have given this a lot of consideration. We may eventually support PPTs but I am not sure it will be a perfect solution for storyboards. Many teams that use PPTs for storyboards place a lot of content off the slide (extra images, notes to SME, developer notes, etc.); these would not be displayed when the PPT is converted :-/
If you're developing eLearning, you could import your PPT and videos into your development software (Storyline, i.e.) and publish a SCORM course for upload on RML.
Though I agree a bit with James's reply, when you use RME a lot for your review process it makes sense to also use it for video. However, the advantage would be that if you pause a video you can add a comment on the time where you paused instead of a step. When you add in Storlyine, you also cannot comment on a frame / time in the video easily.
For PPTX I am not so sure about reviewing purposed as you also have a Notes Field and comments in MS Office documents. Especially for Office 365 with OneDrive, you can really collaborate now.
But for video's it is interesting.
We do a lot of client reviews of videos and PPTs. It would be great to be able to upload these types of files for issue tracking.