Apps do malfunction once in a while. Or you don't want your app to show up in the multitasking pane (for example, I force-quit a game so my students don't see it when I use multitasking in class). This isn't something I do very often, but there definitely is a need for it.
I understand quitting an app if there's a problem, but since it happens every once in a while, it shouldn't be annoying. Also, yes I read that you can swipe up again to quit them.