Ideal scenario would be ONE COLLECTION with a DUAL LAYOUT capability. ie: if the collection is marked as a dual collection, the layout for device is used in that instance.
Currently you have two top-level navigation collections ('default phone' and 'default tablet'), which then include different collections with different layouts. Makes for MORE work for magazine teams. Yes, one article can be used across devices if it is responsive HTML, or a flat PDF (noooo!), but if its an Indesign article, then this won't be optimised for device unless you are tablet, or phone, only.
if you were to use one collection across both devices with one layout - then you'd be limited to what you can do on device - ie: if you design for tablet, the design won't look right on mobile. And vice versa.
If the idea is to help with automation, and resource, and also to create a slicker product, then a dual layout option for all collections would be a start!
I wouldn't be expecting this within Q1 2016. Anyone confirm that?