From a UI designer's standpoint, there are two main reasons for having the tasks in that bar:
1) Emergency task killer. When Jobs made that remark about "If you have a task manager you failed", he really meant, "If the user knows it's a task manager, you failed".
Previously the iOS paradigm was that clicking the Home button was the task manager, since it could stop a runaway app. Clever, simple and hidden from the user.
When Apple added the ability for apps to keep running after the Home button was clicked, that paradigm became obsolete, and another method was needed for the user to kill apps if needed without resetting the whole device. Thus the presence of the (X) kill flags.
2) Back button replacement. They're partly put there in an attempt to make up for the lack of UI context in iOS. In other words, the missing Back button that almost every other device has.
Many apps we use open other apps. For years on the iPhone, the only way to get back to the previous app was to hit the Home button, then remember, find and relaunch the previous app... hoping that it kept its state. This can be a real pain, and spoils some of the power of using multiple non-integrated apps.
On other devices, you simply click its Back button and you return back up the calling chain of apps. This makes using multiple apps quicker and easier.
So Apple, having painted themselves into a corner button-wise, is using the recently used app bar to get around the lack of a Back button. Three clicks to replace one Back click.