What do you find it handy for? iOS does everything automatically for you, and you should be happy that your apps always have resources they need, never unnecessarily waste CPU/memory/battery and you never have to manage them manually.
The problem in proposing such feature is that it would do almost nothing, because pretty much all apps you see in "multitasking" are already closed, and you see just a screenshot of what they looked like when they were open last time.
That is a beautiful dream but it's commonly known that removing apps from the recently used apps list does release memory. The dream also relies on applications that follow the rules and release as much memory as possible when moved to the background.
Here's the documentation in the iOS App Programming Guide.
The important part is:
The app is in the background but is not executing code. The system moves apps to this state automatically and does not notify them before doing so. While suspended, an app remains in memory but does not execute any code.
When a low-memory condition occurs, the system may purge suspended apps without notice to make more space for the foreground app.