I'm not suggesting to convert this app to a generic music player, I just want a feature that makes using it more intuitive. The back button not taking you back is akin to the power button working as a play/pause button, or the volume buttons playing the previous/next song. You could get used to it though, it's fine, because you can still lock your screen by exiting the app and clicking the power button, and you can still control the volume with the knob on another screen. And it makes the app more unique. Why not implement this?
You really don't need to advertise PA to me, I bought it and I use it daily. But in case it wasn't obvious, it's not perfect. Otherwise the developer(s) ought to freeze everything in order to avoid accidentally ruining it. No, my suggestion would not ruin Poweramp if implemented, and it would not make it "just like the other players". There are no advantages whatsoever to having the back button work the way it does now, but again, I agree that it should be toggleable because some people are used to it. There is no way to return to the previous screen from the player UI, so at the moment it's just limiting & counterintuitive behavior that makes Poweramp a bit worse and less powerful than other players, in this specific case.