The nature of using a mobile device for learning is that you’re likely to get interrupted – after all phones were designed primarily to be for phone calls (and are used primarily for text messages). Also learners may well be accessing the learning in situation where they might get interrupted e.g. the train arrives or the boarding call for the plane goes out. If you want learners to resume learning after that interruption then bookmarking is key. If completion is one of your metrics then bookmarking is essential, don’t build anything mobile without it!