BTW, I'm not suggesting this is necessarily the best approach to handling groups for ODK Collect; it is quite a different UI metaphor than what ODK enumerators have become used to (its fairly natural for iOS users because a lot of iOS apps take a similar approach to navigate up and down hierarchies of lists, especially the ubiqutious Settings app).
Personally speaking, I rather like being able to jump around a form easily and fill in things where and when as I prefer, rather than being forced into a predetermined linear process (but whether this freedom is good or bad for enumerators is another question...) Indeed, data availability may require that certain parts of the form be filled in out-of-order as circumstances dictate. So it is nice to be able to do this without having to 'back-track' thru the form step by step, only to have to retrace each step later.
With both current screen sizes and resolutions being much bigger than they were (and getting bigger each year), it could be time to start thinking about offering alternative UI metaphors for Collect, ones perhaps better optimized to the vastly superior screen real-estate now available with current mobile devices. I think we've moved past having to display one question per page.