If it is so (I don't know) why take advantage of it?
As user, I don't like that an app I wanted to close is still "active".
Also, why is there a system function which allows you to terminate an app? It would be totally unnecessary, or not?
Yes, it is so. And as advantage the app start is much faster because it must not loaded and initialized. And why not, it doesn't matter if the app is loaded or not.
If you don't like it then you should stay away from android because almost all apps have this behaviour.
On my Xperia i have two or three dozen apps open without problems and i reboot the device once in a quarter (max.).
The system function to terminate an app is for faulty apps who hangs.
Speculation: if an app has an exit button which completely terminate the app, the programmer may cover misbehaviours
But now we are completely offtopic.