The same design for all slides in the same workspace, a hosted version for non-developers, each slide has its own public url for easy slide playback, and more ...
Try if you are interested in slidepicker.com (beta)
It's a shame, because I think the idea of MARP is revolutionary, but in practice PowerPoint (or even free alternatives like Google Slides) is easier to use.
Be sure to view page source.
Having modifiable source inside the html is a great idea. I'll still need to have a compile workflow to add image while keeping a standalone produced html file (and some other feature such as live-preview), but I'll definitely add this as a possible workflow.