Just curious, why should activities only be treated as entry points to the app? What are the downsides of the only-use-fragments-if-you-need-to approach?
Fragments are one option. Another is just to use regular Views. There are libraries to help you with this such as Conductor.
As for Activities, they are usually overkill for most screens. They don't allow for seamless transitions between screens and prevent sharing UI components across screens (e.g. a Toolbar). Fragments and/or regular views have the added flexibility of being able to use more than one at a time and allows for easy re-use and view composition.
Like I said in my previous comment, the only really good reason to create additional Activities is to provide additional entry points into your app. Otherwise Fragments or Views will suffice and provide greater flexibility and improve the UX in many cases.
For one, the backstack story for Fragments is a lot more sane than whole "Task" concept for activities (The backstack for fragments is a list of fragments you can manipulate and debug very easily).
As for Activities, they are usually overkill for most screens. They don't allow for seamless transitions between screens and prevent sharing UI components across screens (e.g. a Toolbar). Fragments and/or regular views have the added flexibility of being able to use more than one at a time and allows for easy re-use and view composition.
Like I said in my previous comment, the only really good reason to create additional Activities is to provide additional entry points into your app. Otherwise Fragments or Views will suffice and provide greater flexibility and improve the UX in many cases.