Snarky as it may be, "you're using your iPhone wrong" is a pretty myopic way to end that sentence.
There are several reasons a user might want to close an app without saving its state. One is to prevent the app from using background APIs. VoIP apps, GPS apps, etc can continue to access several APIs while their states are saved. In many instances, users might not want this.
Another reason is that many users make use of the app manager as an app switcher. This is one of its intended functions, of course. Switching between commonly used apps with a quick double-tap is far easier than swiping through home screens and digging through folders. It's also easier than using Spotlight.
In this scenario, when I open the app manager why should I have to swipe past the Settings app, the Phone app, the Weather app, the App Store, the Clock app, etc... just to get to the more commonly used app I'm looking for? (Why would I ever want my iPhone to save the Clock app's state in the first place?) The result, for someone who uses the app manager in this way, is a poor user experience.
And this poor user experience is easily avoidable: "How about a long-tap on the home button to close an app, fellas?"