This. The back button is one of the most powerful feature of android. It has always been great for navigating between apps even though it is a bit confusing. Adding a _second_ back button (a third if you count the home button as a long jump back) was the worst decision the android team ever made.

I mean, I understand where they were coming from. When a user jumps between apps he or she might want to be able to navigate inside the apps not just between the apps. However, a single back button with an already too flexible implementation was confusing enough that adding even more navigation options was a terrible decision. One back button forced apps builders to make their app usable within a single thread of history which is a Good Thing.

When a user jumps from app to app and suddenly decides she doesn't want to follow the current line of history, it is not unreasonable, and much less confusing, to make her go through the device home screen to get access to the start screen of the app and its full navigation tree.

When an app opens another app it should be only to give access to a particular section of the second app that is relevant to the first app.

A single back button also helped with the recent apps list mismatches. The mismatches happen when an app is opened from another one and happened less when this type of navigation was for limited specific actions that brought you back to the original app.