First, it probably takes 10 seconds or less to close 20 apps.
Second, why close 20 apps? What is the point.
OMG...let it go. We do not live in a perfect world where apps do what they're supposed to and stop using resources once they're closed. A rogue GPS can keep using location services even when it's not supposed to, DropBox may continue loading in the background after you 'close' it (yes, I know its limited to 10 minutes or whatever, but it will still go against your data plan).
That is the point. Just because you don't find that feature useful doesn't make it unwanted for others. It's like I could careless about Control Center but I get it benefits others.